Tetra Tech (TTEK) 10-K 20 Nov 13 2013 FY Annual report Financial data

Document_and_Entity_Informatio

Document and Entity Information (USD $)	12 Months Ended
In Billions, except Share data, unless otherwise specified	Sep. 29, 2013	Nov. 11, 2013	Mar. 28, 2013
Document and Entity Information	'	'	'
Entity Registrant Name	'TETRA TECH INC	'	'
Entity Central Index Key	'0000831641	'	'
Document Type	'10-K	'	'
Document Period End Date	29-Sep-13	'	'
Amendment Flag	'false	'	'
Current Fiscal Year End Date	'--09-29	'	'
Entity Well-known Seasoned Issuer	'Yes	'	'
Entity Voluntary Filers	'No	'	'
Entity Current Reporting Status	'Yes	'	'
Entity Filer Category	'Large Accelerated Filer	'	'
Entity Public Float	'	'	$1.90
Entity Common Stock, Shares Outstanding	'	64,163,177	'
Document Fiscal Year Focus	'2013	'	'
Document Fiscal Period Focus	'FY	'	'

Consolidated_Balance_Sheets

Consolidated Balance Sheets (USD $)	Sep. 29, 2013	Sep. 30, 2012
In Thousands, unless otherwise specified
Current assets:	'	'
Cash and cash equivalents	$129,305	$104,848
Accounts receivable - net	660,847	700,480
Prepaid expenses and other current assets	61,446	48,168
Income taxes receivable	20,044	5,817
Total current assets	871,642	859,313
Property and equipment - net	88,026	74,309
Investments in and advances to unconsolidated joint ventures	2,198	3,279
Goodwill	722,792	635,958
Intangible assets - net	86,929	74,231
Other long-term assets	27,505	23,940
Total assets	1,799,092	1,671,030
Current liabilities:	'	'
Accounts payable	142,813	154,003
Accrued compensation	114,810	128,086
Billings in excess of costs on uncompleted contracts	79,507	90,909
Deferred income taxes	18,170	20,809
Current portion of long-term debt	4,311	2,031
Estimated contingent earn-out liabilities	23,281	35,407
Other current liabilities	100,241	72,549
Total current liabilities	483,133	503,794
Deferred income taxes	30,525	24,268
Long-term debt	203,438	81,047
Long-term estimated contingent earn-out liabilities	58,508	16,132
Other long-term liabilities	24,685	25,922
Commitments and contingencies	'	'
Equity:	'	'
Preferred stock - Authorized, 2,000 shares of $0.01 par value; no shares issued and outstanding at September 29, 2013, and September 30, 2012	'	'
Common stock - Authorized, 150,000 shares of $0.01 par value; issued and outstanding, 64,134 and 63,837 shares at September 29, 2013, and September 30, 2012, respectively	641	638
Additional paid-in capital	443,099	433,009
Accumulated other comprehensive income	1,858	31,017
Retained earnings	552,165	554,306
Tetra Tech stockholders' equity	997,763	1,018,970
Noncontrolling interests	1,040	897
Total equity	998,803	1,019,867
Total liabilities and equity	$1,799,092	$1,671,030

Consolidated_Balance_Sheets_Pa

Consolidated Balance Sheets (Parenthetical) (USD $)	Sep. 29, 2013	Sep. 30, 2012
In Thousands, except Per Share data, unless otherwise specified
Consolidated Balance Sheets	'	'
Preferred stock, Authorized shares	2,000	2,000
Preferred stock, par value (in dollars per share)	$0.01	$0.01
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, Authorized shares	150,000	150,000
Common stock, par value (in dollars per share)	$0.01	$0.01
Common stock, shares issued	64,134	63,837
Common stock, shares outstanding	64,134	63,837

Consolidated_Statements_of_Ope

Consolidated Statements of Operations (USD $)	12 Months Ended
In Thousands, except Per Share data, unless otherwise specified	Sep. 29, 2013	Sep. 30, 2012	Oct. 02, 2011
Consolidated Statements of Operations	'	'	'
Revenue	$2,613,755	$2,711,075	$2,573,144
Subcontractor costs	-588,923	-689,005	-780,817
Other costs of revenue	-1,757,842	-1,663,065	-1,454,374
Selling, general and administrative expenses	-199,732	-210,970	-193,286
Contingent consideration - fair value adjustments	9,560	19,246	1,755
Impairment of goodwill	-56,600	-914	'
Operating income	20,218	166,367	146,422
Interest income	1,003	873	879
Interest expense	-8,689	-6,444	-6,809
Income before income tax expense	12,532	160,796	140,492
Income tax expense	-14,038	-56,064	-47,510
Net income (loss) including noncontrolling interests	-1,506	104,732	92,982
Net income attributable to noncontrolling interests	-635	-352	-2,943
Net income (loss) attributable to Tetra Tech	($2,141)	$104,380	$90,039
Net income (loss) attributable to Tetra Tech per share:	'	'	'
Basic (in dollars per share)	($0.03)	$1.65	$1.45
Diluted (in dollars per share)	($0.03)	$1.63	$1.43
Weighted-average common shares outstanding:	'	'	'
Basic (in shares)	64,544	63,217	62,053
Diluted (in shares)	64,544	63,934	62,775

Consolidated_Statements_of_Com

Consolidated Statements of Comprehensive Income (Loss) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Sep. 29, 2013	Sep. 30, 2012	Oct. 02, 2011
Consolidated Statements of Comprehensive Income (Loss)	'	'	'
Net income (loss) including noncontrolling interests	($1,506)	$104,732	$92,982
Other comprehensive income (loss), net of tax	'	'	'
Foreign currency translation adjustments	-28,817	26,486	-13,955
Gain (loss) on cash flow hedge valuations	-389	-194	438
Other comprehensive income (loss), net of tax	-29,206	26,292	-13,517
Comprehensive income (loss) including noncontrolling interests	-30,712	131,024	79,465
Net income attributable to noncontrolling interests	-635	-352	-2,943
Foreign currency translation adjustments, net of tax	47	-29	-492
Comprehensive income attributable to noncontrolling interests	-588	-381	-3,435
Comprehensive income (loss) attributable to Tetra Tech	($31,300)	$130,643	$76,030

Consolidated_Statements_of_Equ

Consolidated Statements of Equity (USD $)	Total	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income	Retained Earnings	Total Tetra Tech Equity	Non-Controlling Interests	Comprehensive Income
In Thousands, unless otherwise specified
BALANCE at Oct. 03, 2010	$748,133	$618	$368,865	$18,763	$359,887	$748,133	'	'
BALANCE (in shares) at Oct. 03, 2010	'	61,755	'	'	'	'	'	'
Comprehensive income, net of tax:	'	'	'	'	'	'	'	'
Net income (loss)	92,982	'	'	'	90,039	90,039	2,943	92,982
Foreign currency translation adjustments	-13,955	'	'	-14,447	'	-14,447	492	-13,955
Gain (loss) on cash flow hedge valuations	438	'	'	438	'	438	'	438
Comprehensive income (loss) including noncontrolling interests	79,465	'	'	'	'	76,030	3,435	79,465
Adjustments for consolidation of variable interest entities	670	'	'	'	'	'	670	'
Noncontrolling interest from business acquisitions	438	'	'	'	'	'	438	'
Acquisition of noncontrolling interests	4,567	'	6,883	'	'	6,883	-2,316	'
Distributions paid to noncontrolling interests	-1,702	'	'	'	'	'	-1,702	'
Stock-based compensation	10,582	'	10,582	'	'	10,582	'	'
Stock options exercised	8,004	4	8,000	'	'	8,004	'	'
Stock options exercised (in shares)	'	443	'	'	'	'	'	'
Shares issued for Employee Stock Purchase Plan	5,249	3	5,246	'	'	5,249	'	'
Shares issued for Employee Stock Purchase Plan (in shares)	'	297	'	'	'	'	'	'
Tax expense (benefit) for stock options	-156	'	-156	'	'	-156	'	'
BALANCE at Oct. 02, 2011	855,250	625	399,420	4,754	449,926	854,725	525	'
BALANCE (in shares) at Oct. 02, 2011	'	62,495	'	'	'	'	'	'
Comprehensive income, net of tax:	'	'	'	'	'	'	'	'
Net income (loss)	104,732	'	'	'	104,380	104,380	352	104,732
Foreign currency translation adjustments	26,486	'	'	26,457	'	26,457	29	26,486
Gain (loss) on cash flow hedge valuations	-194	'	'	-194	'	-194	'	-194
Comprehensive income (loss) including noncontrolling interests	131,024	'	'	'	'	130,643	381	131,024
Distributions paid to noncontrolling interests	-9	'	'	'	'	'	-9	'
Stock-based compensation	10,839	'	10,839	'	'	10,839	'	'
Stock options exercised	17,535	10	17,525	'	'	17,535	'	'
Stock options exercised (in shares)	'	1,053	'	'	'	'	'	'
Shares issued for Employee Stock Purchase Plan	5,300	3	5,297	'	'	5,300	'	'
Shares issued for Employee Stock Purchase Plan (in shares)	'	289	'	'	'	'	'	'
Tax expense (benefit) for stock options	-72	'	-72	'	'	-72	'	'
BALANCE at Sep. 30, 2012	1,019,867	638	433,009	31,017	554,306	1,018,970	897	'
BALANCE (in shares) at Sep. 30, 2012	'	63,837	'	'	'	'	'	'
Comprehensive income, net of tax:	'	'	'	'	'	'	'	'
Net income (loss)	-1,506	'	'	'	-2,141	-2,141	635	-1,506
Foreign currency translation adjustments	-28,817	'	'	-28,770	'	-28,770	-47	-28,817
Gain (loss) on cash flow hedge valuations	-389	'	'	-389	'	-389	'	-389
Comprehensive income (loss) including noncontrolling interests	-30,712	'	'	'	'	-31,300	588	-30,712
Distributions paid to noncontrolling interests	-445	'	'	'	'	'	-445	'
Stock-based compensation	8,775	'	8,775	'	'	8,775	'	'
Stock options exercised	14,881	9	14,872	'	'	14,881	'	'
Stock options exercised (in shares)	'	899	'	'	'	'	'	'
Shares issued for Employee Stock Purchase Plan	5,551	3	5,548	'	'	5,551	'	'
Shares issued for Employee Stock Purchase Plan (in shares)	'	253	'	'	'	'	'	'
Stock repurchases	-20,000	-9	-19,991	'	'	-20,000	'	'
Stock repurchases (in shares)	'	-855	'	'	'	'	'	'
Tax expense (benefit) for stock options	886	'	886	'	'	886	'	'
BALANCE at Sep. 29, 2013	$998,803	$641	$443,099	$1,858	$552,165	$997,763	$1,040	'
BALANCE (in shares) at Sep. 29, 2013	'	64,134	'	'	'	'	'	'

Consolidated_Statements_of_Cas

Consolidated Statements of Cash Flows (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Sep. 29, 2013	Sep. 30, 2012	Oct. 02, 2011
Cash flows from operating activities:	'	'	'
Net income (loss) including noncontrolling interests	($1,506)	$104,732	$92,982
Adjustments to reconcile net income (loss) to net cash from operating activities:	'	'	'
Depreciation and amortization	62,605	56,902	55,684
Loss on settlement of foreign currency forward contract	270	286	293
Equity in income of unconsolidated joint ventures	-3,461	-2,916	-4,877
Distributions of earnings from unconsolidated joint ventures	4,458	3,194	4,802
Stock-based compensation	8,775	10,839	10,582
Excess tax benefits from stock-based compensation	-886	-624	-104
Deferred income taxes	-11,468	-5,512	1,720
Provision for doubtful accounts	13,818	4,768	3,733
Impairment of goodwill	56,600	914	'
Fair value adjustments to contingent consideration	-9,560	-19,246	-1,755
Fair value adjustment to assets held for sale	'	3,437	'
Foreign exchange (gain) loss	754	-139	1,288
Lease termination costs and related asset impairment	7,188	1,261	1,281
(Gain) loss on disposal of property and equipment	-287	191	-231
Changes in operating assets and liabilities, net of effects of business acquisitions:	'	'	'
Accounts receivable	87,367	-39,960	2,046
Prepaid expenses and other assets	-11,782	26,284	-28,324
Accounts payable	-34,191	-14,529	-34,013
Accrued compensation	-16,385	15,678	11,157
Billings in excess of costs on uncompleted contracts	-16,830	2,425	-1,669
Other liabilities	21,489	7,371	6,475
Income taxes receivable/payable	-19,218	2,665	10,553
Net cash provided by operating activities	137,750	158,021	131,623
Cash flows from investing activities:	'	'	'
Capital expenditures	-27,545	-25,106	-18,901
Payments for business acquisitions, net of cash acquired	-171,329	-55,014	-269,996
Payment in settlement of foreign currency forward contract	-4,177	-4,192	-4,216
Receipt in settlement of foreign currency forward contract	3,907	3,906	3,923
Changes in restricted cash	470	'	-5,000
Investments in unconsolidated joint ventures	-20	-430	-530
Proceeds from sale of property and equipment	2,089	1,037	879
Net cash used in investing activities	-196,605	-79,799	-293,841
Cash flows from financing activities:	'	'	'
Payments on long-term debt	-171,400	-120,792	-43,047
Proceeds from borrowings	296,389	52,672	67,618
Payments of earn-out liabilities	-33,672	-18,055	'
Payment of debt issuance cost	-2,136	'	'
Distributions paid to noncontrolling interests	-445	-9	-1,702
Excess tax benefits from stock-based compensation	886	624	104
Repurchases of common stock	-20,000	'	'
Net proceeds from issuance of common stock	15,993	18,166	8,378
Net cash provided by (used in) financing activities	85,615	-67,394	31,351
Effect of foreign exchange rate changes on cash	-2,303	3,526	428
Net increase (decrease) in cash and cash equivalents	24,457	14,354	-130,439
Cash and cash equivalents at beginning of year	104,848	90,494	220,933
Cash and cash equivalents at end of year	129,305	104,848	90,494
Cash paid during the year for:	'	'	'
Interest	5,049	5,279	4,226
Income taxes, net of refunds received	$35,796	$58,126	$33,715

Description_of_Business

Description of Business	12 Months Ended
	Sep. 29, 2013
Description of Business	'
Description of Business	'
	1.           Description of Business
	We are a leading provider of consulting, engineering, program management, construction management, construction and technical services that focuses on addressing fundamental needs for water, the environment, energy, infrastructure and natural resources. We are a full-service company that leads with science. We typically begin at the earliest stage of a project by identifying technical solutions to problems and developing execution plans tailored to our clients' needs and resources. Our solutions may span the entire life cycle of consulting and engineering projects and include applied science, research and technology, engineering, design, construction management, construction, operations and maintenance, and information technology.

Basis_of_Presentation_and_Prep

Basis of Presentation and Preparation	12 Months Ended
	Sep. 29, 2013
Basis of Presentation and Preparation	'
Basis of Presentation and Preparation	'
	2.           Basis of Presentation and Preparation
	Principles of Consolidation and Presentation.    The consolidated financial statements include our accounts and those of joint ventures of which we are the primary beneficiary. Certain prior year amounts for Tetra Tech, Inc. and its reportable segments have been revised to conform to the current year presentation. In the first quarter of fiscal 2013, we implemented a reorganization of our operations to improve future growth and profitability, including the consolidation and realignment of certain operating activities to achieve efficiencies in our segment management. This reorganization included the elimination of the EAS reportable segment, and the re-assignment of its operations to the ECS and TSS segments (see Note 18, "Reportable Segments" for further discussion). All significant intercompany balances and transactions have been eliminated in consolidation.
	Fiscal Year.    We report results of operations based on 52 or 53-week periods ending on the Sunday nearest September 30. Fiscal years 2013, 2012 and 2011 each contained 52 weeks.
	Use of Estimates.    The preparation of financial statements in conformity with U.S. GAAP requires us to make estimates and assumptions. These estimates and assumptions affect the amounts reported in our consolidated financial statements and accompanying notes. Although such estimates and assumptions are based on management's best knowledge of current events and actions we may take in the future, actual results could differ materially from those estimates.
	Revenue Recognition and Contract Costs.    We recognize revenue for most of our contracts using the percentage-of-completion method, primarily based on contract costs incurred to date compared to total estimated contract costs. We generally utilize the cost-to-cost approach to estimate the progress towards completion in order to determine the amount of revenue and profit to recognize. Revenue and cost estimates for each significant contract are reviewed and reassessed quarterly. Changes in those estimates could result in recognition of cumulative catch-up adjustments to the contract's inception-to-date revenue, costs and profit in the period in which such changes are made. Changes in revenue and cost estimates could also result in a projected loss that would be recorded immediately in earnings. For fiscal years 2013, 2012 and 2011, we recognized net favorable (unfavorable) operating income adjustments of ($40.1) million, $0.5 million and ($8.6) million, respectively, due to changes in estimates.
	Certain of our contracts are service-related contracts, such as providing operations and maintenance services or a variety of technical assistance services. Our service contracts are accounted for using the proportional performance method under which revenue is recognized in proportion to the number of service activities performed, in proportion to the direct costs of performing the service activities, or evenly across the period of performance depending upon the nature of the services provided.
	We recognize revenue for work performed under three major types of contracts: fixed-price, time-and-materials and cost-plus.
	Fixed-Price.    We enter into two major types of fixed-price contracts: FFP and FPPU. Under FFP contracts, our clients pay us an agreed fixed-amount negotiated in advance for a specified scope of work. We generally recognize revenue on FFP contracts using the percentage-of-completion method. If the nature or circumstances of the contract prevent us from preparing a reliable estimate at completion, we will delay profit recognition until adequate information about the contract's progress becomes available. Under our FPPU contracts, clients pay us a set fee for each service or production transaction that we complete. Accordingly, we recognize revenue under FPPU contracts as we complete the related service or production transactions, generally using the proportional performance method.
	Time-and-Materials.    Under time-and-materials contracts, we negotiate hourly billing rates and charge our clients based on the actual time that we spend on a project. In addition, clients reimburse us for our actual out-of-pocket costs for materials and other direct incidental expenditures that we incur in connection with our performance under the contract. The majority of our time-and-material contracts are subject to maximum contract values and, accordingly, revenue under these contracts is generally recognized under the percentage-of-completion method. However, time and materials contracts that are service-related contracts are accounted for utilizing the proportional performance method. Revenue on contracts that are not subject to maximum contract values is recognized based on the actual number of hours we spend on the projects plus any actual out-of-pocket costs of materials and other direct incidental expenditures that we incur on the projects. Our time-and-materials contracts also generally include annual billing rate adjustment provisions.
	Cost-Plus.    Under cost-plus contracts, we are reimbursed for allowable or otherwise defined costs incurred plus a fee or mark-up. The contracts may also include incentives for various performance criteria, including quality, timeliness, ingenuity, safety and cost-effectiveness. In addition, our costs are generally subject to review by our clients and regulatory audit agencies, and such reviews could result in costs being disputed as non-reimbursable under the terms of the contract. Revenue for cost-plus contracts is recognized at the time services are performed based upon the amounts we expect to realize using the percentage-of-completion method. Revenue is not recognized for non-recoverable costs. Performance incentives are included in our estimates of revenue when their realization is reasonably assured.
	If estimated total costs on any contract indicate a loss, we recognize the entire estimated loss in the period the loss becomes known. The cumulative effect of revisions to revenue; estimated costs to complete contracts, including penalties, incentive awards, change orders, claims, liquidated damages, anticipated losses, and other revisions are recorded in the period in which the revisions are identified and the loss can be reasonably estimated. Such revisions could occur in any reporting period and the effects may be material depending on the size of the project or the adjustment.
	Once contract performance is underway, we may experience changes in conditions, client requirements, specifications, designs, materials and expectations regarding the period of performance. Such changes are "change orders" and may be initiated by us or by our clients. In many cases, agreement with the client as to the terms of change orders is reached prior to work commencing; however, sometimes circumstances require that work progress without obtaining client agreement. Revenue related to change orders is recognized as costs are incurred. Change orders that are unapproved as to both price and scope are evaluated as claims.
	Claims are amounts in excess of agreed contract prices that we seek to collect from our clients or other third parties for delays, errors in specifications and designs, contract terminations, change orders in dispute or unapproved as to both scope and price, or other causes of unanticipated additional costs. Revenue on claims is recognized only to the extent that contract costs related to the claims have been incurred and when it is probable that the claim will result in a bona fide addition to contract value that can be reliably estimated. No profit is recognized on a claim until final settlement occurs. This can lead to a situation in which costs are recognized in one period and revenue is recognized in a subsequent period when a client agreement is obtained or a claims resolution occurs.
	Cash and Cash Equivalents.    Cash and cash equivalents include all highly liquid investments with maturities of 90 days or less at the date of purchase. Restricted cash of $4.5 million and $5.0 million was included in "Prepaid expenses and other current assets" on consolidated balance sheets at fiscal 2013 and 2012 year-ends, respectively. For cash held by our consolidated joint ventures, see Note 16, "Joint Ventures."
	Insurance Matters, Litigation and Contingencies.    In the normal course of business, we are subject to certain contractual guarantees and litigation. In addition, we maintain insurance coverage for various aspects of our business and operations. We record in our consolidated balance sheets amounts representing our estimated liability for these legal and insurance obligations. We include any adjustments to these liabilities in our consolidated results of operations.
	Accounts Receivable – Net.    Net accounts receivable is primarily comprised of billed and unbilled accounts receivable, contract retentions and allowances for doubtful accounts. Billed accounts receivable represent amounts billed to clients that have not been collected. Unbilled accounts receivable represent revenue recognized but not yet billed pursuant to contract terms or billed after the period end date. Most of our unbilled receivables at September 29, 2013 are expected to be billed and collected within 12 months. Unbilled accounts receivable also include amounts related to requests for equitable adjustment to contracts that provide for price redetermination primarily with the U.S. federal government. These amounts are recorded only when they can be reliably estimated and realization is probable. Contract retentions represent amounts withheld by clients until certain conditions are met or the project is completed, which may be several months or years. Allowances for doubtful accounts represent the amounts that may become uncollectible or unrealizable in the future. We determine an estimated allowance for uncollectible accounts based on management's judgment regarding our operating performance related to the adequacy of the services performed and delivered, the status of change orders and claims, our experience settling change orders and claims, and the financial condition of our clients. Billings in excess of costs on uncompleted contracts represent the amounts of cash collected from clients and billings to clients on contracts in advance of work performed and revenue recognized. The majority of these amounts will be earned within 12 months.
	Property and Equipment.    Property and equipment are recorded at cost and are depreciated over their estimated useful lives using the straight-line method. When property and equipment are retired or otherwise disposed of, the cost and accumulated depreciation are removed from our consolidated balance sheets and any resulting gain or loss is reflected in our consolidated statements of operations. Expenditures for maintenance and repairs are expensed as incurred. Generally, estimated useful lives range from three to ten years for equipment, furniture and fixtures. Buildings are depreciated over periods not exceeding 40 years. Leasehold improvements are amortized on a straight-line basis over the shorter of their estimated useful lives or the length of the lease.
	Long-Lived Assets.    Our policy regarding long-lived assets is to evaluate the recoverability of our assets when the facts and circumstances suggest that the assets may be impaired. This assessment is performed based on the estimated undiscounted cash flows compared to the carrying value of the assets. If the future cash flows (undiscounted and without interest charges) are less than the carrying value, a write-down would be recorded to reduce the related asset to its estimated fair value.
	We recognize a liability for contract termination costs associated with an exit activity for costs that will continue to be incurred under a lease for its remaining term without economic benefit to us, initially measured at its fair value at the cease-use date. The fair value is determined based on the remaining lease rentals, adjusted for the effects of any prepaid or deferred items recognized under the lease, and reduced by estimated sublease rentals.
	Business Combinations.    The cost of an acquired company is assigned to the tangible and intangible assets purchased and the liabilities assumed on the basis of their fair values at the date of acquisition. The determination of fair values of assets and liabilities acquired requires us to make estimates and use valuation techniques when a market value is not readily available. Any excess of purchase price over the fair value of net tangible and intangible assets acquired is allocated to goodwill. Goodwill typically represents the value paid for the assembled workforce and enhancement of our service offerings. Transaction costs associated with business combinations are expensed as they are incurred.
	Goodwill and Intangible Assets.    Goodwill represents the excess of the aggregate purchase price over the fair value of the net assets acquired in a business acquisition. Following an acquisition, we perform an analysis to value the acquired company's tangible and identifiable intangible assets and liabilities. With respect to identifiable intangible assets, we consider backlog, non-compete agreements, client relations, trade names, patents and other assets. We amortize our intangible assets based on the period over which the contractual or economic benefits of the intangible assets are expected to be realized. We assess the recoverability of the unamortized balance of our intangible assets when indicators of impairment are present based on expected future profitability and undiscounted expected cash flows and their contribution to our overall operations. Should the review indicate that the carrying value is not fully recoverable, the excess of the carrying value over the fair value of the intangible assets would be recognized as an impairment loss.
	We test our goodwill for impairment on an annual basis, and more frequently when an event occurs or circumstances indicate that the carrying value of the asset may not be recoverable. We believe the methodology that we use to review impairment of goodwill, which includes a significant amount of judgment and estimates, provides us with a reasonable basis to determine whether impairment has occurred. However, many of the factors employed in determining whether our goodwill is impaired are outside of our control and it is reasonably likely that assumptions and estimates will change in future periods. These changes could result in future impairments.
	We perform our annual goodwill impairment review at the beginning of our fiscal fourth quarter. Our last annual review at July 1, 2013 (i.e., the first day of our fiscal fourth quarter), indicated that we had no impairment of goodwill, and all of our reporting units had estimated fair values that were in excess of their carrying values, including goodwill. In addition, we regularly evaluate whether events and circumstances have occurred that may indicate a potential change in recoverability of goodwill. We perform interim goodwill impairment reviews between our annual reviews if certain events and circumstances have occurred, including a deterioration in general economic conditions, an increased competitive environment, a change in management, key personnel, strategy or customers, negative or declining cash flows, or a decline in actual or planned revenue or earnings compared with actual and projected results of relevant prior periods (See Note 6, "Goodwill and Intangible Assets" for further discussion). We assess goodwill for impairment at the reporting unit level, which is defined as an operating segment or one level below an operating segment, referred to as a component. Our operating segments are the same as our reportable segments and our reporting units for goodwill impairment testing are the components one level below our reportable segments. These components constitute a business for which discrete financial information is available and where segment management regularly reviews the operating results of that component. We aggregate components within an operating segment that have similar economic characteristics.
	The impairment test for goodwill is a two-step process involving the comparison of the estimated fair value of each reporting unit to the reporting unit's carrying value, including goodwill. We estimate the fair value of reporting units based on a comparison and weighting of the income approach, specifically the discounted cash flow method and the market approach, which estimates the fair value our reporting units based upon comparable market prices and recent transactions and also validates the reasonableness of the multiples from the income approach. If the fair value of a reporting unit exceeds its carrying amount, the goodwill of the reporting unit is not considered impaired; therefore, the second step of the impairment test is unnecessary. If the carrying amount of a reporting unit exceeds its fair value, we perform the second step of the goodwill impairment test to measure the amount of impairment loss to be recorded. If our goodwill is impaired, we are required to record a non-cash charge that could have a material adverse effect on our consolidated financial statements.
	Contingent Consideration.    Most of our acquisition agreements include contingent earn-out arrangements, which are generally based on the achievement of future operating income thresholds. The contingent earn-out arrangements are based upon our valuations of the acquired companies and reduce the risk of overpaying for acquisitions if the projected financial results are not achieved. For acquisitions completed prior to fiscal 2010, contingent earn-out payments are accrued as "Estimated earn-out liabilities" when the related operating income thresholds have been achieved, and a corresponding increase in goodwill is recorded. These contingent earn-out payments are reflected as cash flows used in investing activities on the consolidated statements of cash flows in the period paid. At September 29, 2013, there was no contingent consideration remaining for acquisitions completed prior to fiscal 2010 that would be recorded as an addition to goodwill, if earned.
	For acquisitions consummated during or subsequent to fiscal 2010, the fair values of these earn-out arrangements are included as part of the purchase price of the acquired companies on their respective acquisition dates. For each transaction, we estimate the fair value of contingent earn-out payments as part of the initial purchase price and record the estimated fair value of contingent consideration as a liability in "Estimated contingent earn-out liabilities" and "Long-term estimated contingent earn-out liabilities" on the consolidated balance sheets. We consider several factors when determining that contingent earn-out liabilities are part of the purchase price, including the following: (1) the valuation of our acquisitions is not supported solely by the initial consideration paid, and the contingent earn-out formula is a critical and material component of the valuation approach to determining the purchase price; and (2) the former owners of acquired companies that remain as key employees receive compensation other than contingent earn-out payments at a reasonable level compared with the compensation of our other key employees. The contingent earn-out payments are not affected by employment termination.
	We measure our contingent earn-out liabilities at fair value on a recurring basis using significant unobservable inputs classified within Level 3 of the fair value hierarchy. We use a probability weighted discounted income approach as a valuation technique to convert future estimated cash flows to a single present value amount. The significant unobservable inputs used in the fair value measurements are operating income projections over the earn-out period (generally two or three years), and the probability outcome percentages we assign to each scenario. Significant increases or decreases to either of these inputs in isolation would result in a significantly higher or lower liability with a higher liability capped by the contractual maximum of the contingent earn-out obligation. Ultimately, the liability will be equivalent to the amount paid, and the difference between the fair value estimate and amount paid will be recorded in earnings. The amount paid that is less than or equal to the liability on the acquisition date is reflected as cash used in financing activities in our consolidated statements of cash flows. Any amount paid in excess of the liability on the acquisition date is reflected as cash used in operating activities.
	We review and re-assess the estimated fair value of contingent consideration on a quarterly basis, and the updated fair value could differ materially from the initial estimates. Changes in the estimated fair value of our contingent earn-out liabilities related to the time component of the present value calculation are reported in interest expense. Adjustments to the estimated fair value related to changes in all other unobservable inputs are reported in operating income.
	Assets Held for Sale.    Assets that meet the held for sale classification criteria are valued at the lower of their carrying amount or estimated fair value less cost to sell. If the carrying amount of the asset exceeds its estimated fair value less cost to sell, an impairment loss is recognized. Depreciation, depletion and amortization expense is not recorded on assets once they are classified as held for sale.
	Fair Value of Financial Instruments.    We determine the fair values of our financial instruments, including short-term investments, debt instruments and derivative instruments based on inputs or assumptions that market participants would use in pricing an asset or a liability. We categorize our instruments using a valuation hierarchy for disclosure of the inputs used to measure fair value. This hierarchy prioritizes the inputs into three broad levels as follows: Level 1 inputs are quoted prices (unadjusted) in active markets for identical assets or liabilities; Level 2 inputs are quoted prices for similar assets and liabilities in active markets or inputs that are observable for the asset or liability, either directly or indirectly through market corroboration, for substantially the full term of the financial instrument; and Level 3 inputs are unobservable inputs based on our own assumptions used to measure assets and liabilities at fair value. The classification of a financial asset or liability within the hierarchy is determined based on the lowest level input that is significant to the fair value measurement.
	The carrying amounts of cash and cash equivalents, accounts receivable and accounts payable approximate fair values based on their short-term nature. The carrying amounts of our revolving credit facility approximates fair value because the interest rates are based upon variable reference rates (see Note 9, "Long-Term Debt" and Note 14, "Derivative Financial Instruments" for additional disclosure). Certain other assets and liabilities, such as contingent earn-out liabilities, assets held for sale and amounts related to cash-flow hedges, are required to be carried in our consolidated financial statements at fair value.
	Our fair value measurement methods may produce a fair value calculation that may not be indicative of net realizable value or reflective of future fair values. Although we believe our valuation methods are appropriate and consistent with those used by other market participants, the use of different methodologies or assumptions to determine fair value could result in a different fair value measurement at the reporting date.
	Derivative Financial Instruments.    We account for our derivative instruments as either assets or liabilities and carry them at fair value. For derivative instruments that hedge the exposure to variability in expected future cash flows that are designated as cash flow hedges, the effective portion of the gain or loss on the derivative instrument is reported as a component of accumulated other comprehensive income in stockholders' equity and reclassified into income in the same period or periods during which the hedged transaction affects earnings. The ineffective portion of the gain or loss on the derivative instrument, if any, is recognized in current income. To receive hedge accounting treatment, cash flow hedges must be highly effective in offsetting changes to expected future cash flows on hedged transactions.
	The net gain or loss on the effective portion of a derivative instrument that is designated as an economic hedge of the foreign currency translation exposure generated by the re-measurement of certain assets and liabilities denominated in a non-functional currency in a foreign operation is reported in the same manner as a foreign currency translation adjustment. Accordingly, any gains or losses related to these derivative instruments are recognized in current income. Derivatives that do not qualify as hedges are adjusted to fair value through current income.
	Deferred Compensation.    We maintain a non-qualified defined contribution supplemental retirement plan for certain key employees that is accounted for in accordance with applicable authoritative guidance on accounting for deferred compensation arrangements where amounts earned are held in a rabbi trust and invested. Employee deferrals and our match are deposited into a rabbi trust, and the funds are generally invested in individual variable life insurance contracts that we own and are specifically designed to informally fund savings plans of this nature. Our consolidated balance sheets reflect our investment in variable life insurance contracts in "Other long-term assets." Our obligation to participating employees is reflected in "Other long-term liabilities." All income and expenses related to the rabbi trust are reflected in our consolidated statements of operations.
	Income Taxes.    We file a consolidated U.S. federal income tax return and a combined California franchise tax return. In addition, we file other returns that are required in the states, foreign jurisdictions and other jurisdictions in which we do business. We account for certain income and expense items differently for financial reporting and income tax purposes. Deferred tax assets and liabilities are computed for the difference between the financial statement and tax bases of assets and liabilities that will result in taxable or deductible amounts in the future based on enacted tax laws and rates applicable to the periods in which the differences are expected to reverse. In determining the need for a valuation allowance, management reviews both positive and negative evidence, including current and historical results of operations, future income projections and potential tax planning strategies. Based on our assessment, we have concluded that a portion of the deferred tax assets at September 29, 2013 will not be realized.
	According to the authoritative guidance on accounting for uncertainty in income taxes, we may recognize the tax benefit from an uncertain tax position only if it is more likely than not that the tax position will be sustained on examination by the taxing authorities based on the technical merits of the position. The tax benefits recognized in the financial statements from such a position should be measured based on the largest benefit that has a greater than 50% likelihood of being realized upon ultimate settlement. This guidance also addresses de-recognition, classification, interest and penalties on income taxes, accounting in interim periods and disclosure requirements for uncertain tax positions.
	In the fourth quarter of fiscal 2013, we identified and corrected errors relating to prior years that increased both income tax expense and our net loss for the year by $3.3 million. The out-of-period adjustments related to book versus tax valuations of fixed assets and acquired intangible assets. Had all adjustments been recorded in their appropriate periods, net loss attributable to Tetra Tech for fiscal 2013 would have increased to net income of $1.1 million. We do not believe that the accounting errors are material to our 2013 annual results or to our previously issued annual or interim financial statements. As a result, we did not adjust any prior period amounts.
	Concentration of Credit Risk.    Financial instruments that subject us to credit risk consist primarily of cash and cash equivalents and net accounts receivable. In the event that we have surplus cash, we place our temporary cash investments with lower risk financial institutions and, by policy, limit the amount of investment exposure to any one financial institution. Approximately 22% of accounts receivable were due from various agencies of the U.S. federal government at fiscal 2013 year-end. The remaining accounts receivable are generally diversified due to the large number of organizations comprising our client base and their geographic dispersion. We perform ongoing credit evaluations of our clients and maintain an allowance for potential credit losses. Approximately 47%, 26% and 27% of our fiscal 2013 revenue was generated from our U.S government, U.S. commercial and international clients, respectively (see Note 18, "Reportable Segments" for more information).
	Foreign Currency Translation.    We determine the functional currency of our foreign operating units based upon the primary currency in which they operate. These operating units maintain their accounting records in their local currency, primarily CAD. Where the functional currency is not the U.S. dollar, translation of assets and liabilities to U.S. dollars is based on exchange rates at the balance sheet date. Translation of revenue and expenses to U.S. dollars is based on the average rate during the period. Translation gains or losses are reported as a component of other comprehensive income (loss). Gains or losses from foreign currency transactions are included in results of operations, with the exception of intercompany foreign transactions that are considered long-term investments, which are recorded in "Accumulated other comprehensive income" on the consolidated balance sheets.
	Recently Adopted and Issued Accounting Guidance.    In June 2011, the Financial Accounting Standards Board ("FASB") issued new guidance on the presentation of comprehensive income. We are required to present components of net income and other comprehensive income in either one single continuous statement or in two separate consecutive statements. This guidance was effective for us in the first quarter of fiscal 2013 and it did not have an impact on our consolidated financial statements.
	In September 2011, the FASB issued updated guidance to simplify goodwill impairment testing. The amendment permits us to first assess qualitative factors to determine whether it is necessary to perform the two-step quantitative goodwill impairment test. The guidance was effective for us in July 2013 when we perform our annual goodwill impairment test. This guidance did not have an impact on our consolidated financial statements.
	In December 2011, the FASB issued new guidance to enhance disclosures about financial instruments and derivative instruments that are either offset on the statement of financial position or subject to an enforceable master netting arrangement or similar agreement, irrespective of whether they are offset on the statement of financial position. We are required to provide both net and gross information for these assets and liabilities in order to facilitate comparability between financial statements prepared on the basis of U.S. GAAP and financial statements prepared on the basis of International Financial Reporting Standards. This guidance will be effective for us in the first quarter of fiscal 2014 on a retrospective basis. We are currently evaluating the impact on our consolidated financial statements.
	In February 2013, the FASB issued an update to the reporting of reclassifications out of accumulated other comprehensive income. We are required to disclose additional information about changes in and significant items reclassified out of accumulated other comprehensive income. The guidance is effective for us in the first quarter of fiscal 2014. We do not expect the adoption of this guidance to have an impact on our consolidated financial statements.
	In July 2013, the FASB issued an update on an inclusion of the Fed Funds Effective Swap as a benchmark interest rate (Overnight Interest Swap Rate) for hedge accounting purposes. This guidance permits the Fed Funds Effective Swap Rate to be used as a U.S. benchmark interest rate for hedge accounting purposes under U.S. GAAP. This guidance will be effective prospectively for qualifying new or redesigned hedging relationships entered into on or after July 17, 2013. We do not expect a material impact to our consolidated financial statements.
	In July 2013, the FASB issued an update on the financial statement presentation of unrecognized tax benefits. We are required to present a liability related to an unrecognized tax benefit as a reduction of a deferred tax asset for a net operating loss carryforward, a similar tax loss or a tax credit carryforward if such settlement is required or expected in the event the uncertain tax position is disallowed. This guidance will be effective for us in the first quarter of fiscal 2015. We do not expect the adoption of this guidance to have an impact on our consolidated financial statements.

Stock_Repurchase_Program

Stock Repurchase Program	12 Months Ended
	Sep. 29, 2013
Stock Repurchase Program	'
Stock Repurchase Program	'
	3.           Stock Repurchase Program
	In June 2013, our Board of Directors authorized the Stock Repurchase Program under which we may currently repurchase up to $100 million of Tetra Tech common stock. In November 2013, our Board of Directors amended the Stock Repurchase Program, effective on November 18, 2013, to revise the pricing parameters and to extend the program through fiscal 2014. Stock repurchases may be made on the open market or in privately negotiated transactions with third parties. Because the repurchases under the Stock Repurchase Program are subject to certain pricing parameters, there is no guarantee as to the exact number of shares that will be repurchased under the program. At September 29, 2013, we had repurchased through open market purchases a total of 855,200 shares at an average price of $23.39 per share, for a total cost of $20.0 million.

Accounts_Receivable_Net

Accounts Receivable - Net	12 Months Ended
	Sep. 29, 2013
Accounts Receivable - Net	'
Accounts Receivable - Net	'
	4.           Accounts Receivable – Net
	Net accounts receivable and billings in excess of costs on uncompleted contracts consisted of the following at September 29, 2013 and September 30, 2012:
		Fiscal Year Ended
		September 29,			September 30,
		2013			2012
		(in thousands)
	Billed	$	375,149		$	362,331
	Unbilled		306,969			355,793
	Contract retentions		23,353			17,908
	Total accounts receivable – gross		705,471			736,032
	Allowance for doubtful accounts		(44,624	)		(35,552	)
	Total accounts receivable – net	$	660,847		$	700,480
	Current billings in excess of costs on uncompleted contracts	$	79,507		$	90,909
	Non-current billings in excess of costs on uncompleted contracts		–			4,410
	Total billings in excess of costs on uncompleted contracts	$	79,507		$	95,319
	Billed accounts receivable represent amounts billed to clients that have not been collected. Unbilled accounts receivable represent revenue recognized but not yet billed pursuant to contract terms or billed after the period end date. Most of our unbilled receivables at September 29, 2013 are expected to be billed and collected within 12 months. Contract retentions represent amounts withheld by clients until certain conditions are met or the project is completed, which may be several months or years. The allowance for doubtful accounts is determined based on a review of client-specific accounts, and contract issues resulting from current events and economic circumstances. Billings in excess of costs on uncompleted contracts represent the amount of cash collected from clients and billings to clients on contracts in advance of revenue recognized. The majority of billings in excess of costs on uncompleted contracts will be earned within 12 months. Non-current billings in excess of costs on uncompleted contracts are reported as part of our "Other long-term liabilities" on our consolidated balance sheets.
	Once contract performance is underway, we may experience changes in conditions, client requirements, specifications, designs, materials and expectations regarding the period of performance. Such changes result in "change orders" and may be initiated by us or by our clients. In many cases, agreement with the client as to the terms of change orders is reached prior to work commencing; however, sometimes circumstances require that work progresses without obtaining client agreement. Unapproved change orders constitute claims in excess of agreed contract prices that we seek to collect from our clients (or other third parties) for delays, errors in specifications and designs, contract terminations, or other causes of unanticipated additional costs. Revenue on claims is recognized when contract costs related to claims have been incurred and when their addition to contract value can be reliably estimated. This can lead to a situation in which costs are recognized in one period and revenue is recognized in a subsequent period such as when client agreement is obtained or a claims resolution occurs.
	Unbilled accounts receivable at September 29, 2013 and September 30, 2012 include approximately $41 million and $21 million, respectively, related to claims, including requests for equitable adjustment, on contracts that provide for price redetermination, primarily with U.S. federal government agencies. We regularly evaluate these claim amounts and record appropriate adjustments to operating earnings when it is probable that the claim will result in a different contract value than the amount previously reliably estimated. We recognized losses of approximately $30.2 million related to the evaluation of collectability of claims in fiscal 2013 primarily related to contractual disputes with government clients. No losses related to claims were recognized in fiscal 2012.
	Billed accounts receivable related to U.S. federal government contracts were $50.5 million and $65.9 million at September 29, 2013 and September 30, 2012, respectively. U.S. federal government unbilled receivables, net of progress payments, were $79.3 million and $100.4 million at September 29, 2013 and September 30, 2012, respectively. Other than the U.S. federal government, no single client accounted for more than 10% of our accounts receivable at September 29, 2013 and September 30, 2012.

Mergers_and_Acquisitions

Mergers and Acquisitions	12 Months Ended
	Sep. 29, 2013
Mergers and Acquisitions	'
Mergers and Acquisitions	'
	5.           Mergers and Acquisitions
	At the beginning of the first quarter of fiscal 2011, we acquired all of the outstanding capital stock of BPR, Inc. ("BPR"), a Canadian scientific and engineering services firm that provides multidisciplinary consulting and engineering support for water, energy, industrial plants, buildings and infrastructure projects. This acquisition further expanded our geographic presence in Eastern Canada, and enabled us to provide clients with additional services throughout Canada. BPR is part of our ECS segment. The estimated fair value of the purchase price was $185.7 million as of the acquisition date, of which payments of $157.0 million were financed with borrowings under our credit facility and available cash resources, and $28.7 million was the estimated fair value of contingent earn-out liabilities on acquisition with a maximum of $39.2 million upon the achievement of specified financial objectives over a two-year period from the acquisition date. The goodwill related to the BPR acquisition represented the value paid for the assembled work force, the international geographic presence in Eastern Canada, and engineering and consulting expertise. The following table summarizes the estimated fair values of the assets acquired and liabilities assumed as of the date of acquisition:
		Amount
		(in thousands)
	Current assets	$	77,698
	Property and equipment		7,178
	Goodwill		128,140
	Intangible and other assets		36,988
	Current liabilities		(42,481	)
	Long-term deferred taxes		(9,622	)
	Noncontrolling interests		(12,222	)
	Net assets acquired	$	185,679
	In fiscal 2011, we made other acquisitions that enhanced our service offerings and expanded our geographic presence in the ECS and TSS segments. The aggregate purchase price for these acquisitions was approximately $100 million as of the respective acquisition dates. Of this amount, $68.7 million was paid to the sellers, $4.5 million was accrued in accordance to the purchase agreements, and $26.9 million was the estimated fair value of contingent earn-out obligations with an aggregate maximum of $32.3 million upon the achievement of specified financial objectives.
	In fiscal 2012, we made acquisitions that enhanced our service offerings and expanded our geographic presence in our ECS and TSS segments. The aggregate purchase price for these acquisitions was $63.2 million as of the respective acquisition dates. Of this amount, $42.2 million was paid to the sellers, $2.0 million was accrued in accordance to the purchase agreements, and $19.0 million was the estimated fair value of contingent earn-out obligations with an aggregate maximum of $20.0 million upon the achievement of specified financial objectives.
	On December 31, 2012, we acquired American Environmental Group, Ltd. ("AEG"), headquartered in Richfield, Ohio. AEG provides environmental, design, construction and maintenance services primarily to solid and hazardous waste, environmental, energy and utility clients. On January 28, 2013, we acquired Parkland Pipeline Contractors Ltd., Parkland Pipeline Equipment Ltd., Park L Projects Ltd. and Parkland Projects Ltd. (collectively, "Parkland"), headquartered in Alberta, Canada. Parkland serves the oil and gas industry in Western Canada, and specializes in the technical support, engineering support and construction of pipelines and oilfield facilities. AEG and Parkland are both included in our RCM segment. We also made other acquisitions that enhanced our service offerings and expanded our geographic presence in our ECS and TSS segments during fiscal 2013. The aggregate fair value of the purchase prices for fiscal 2013 acquisitions was $248.9 million. Of this amount, $171.6 million was paid to the sellers, $2.0 million was recorded as liabilities in accordance with the purchase agreements, and $75.3 million was the estimated fair value of contingent earn-out obligations, with an aggregate maximum of $86.7 million upon the achievement of specified financial objectives.
	Goodwill additions resulting from the above business combinations are primarily attributable to the existing workforce of the acquired companies and synergies expected to arise after the acquisitions. The results of these acquisitions were included on the consolidated financial statements from their respective closing dates. None of the acquisitions were considered material, individually or in the aggregate, for the respective reporting periods. As a result, no pro forma information has been provided. The purchase price allocations related to fiscal 2013 acquisitions are preliminary, and subject to adjustment, based on the valuation and final determination of net assets acquired. We do not believe that any adjustments will have a material effect on the consolidated results of operations.
	At September 29, 2013, there was a total maximum of $101.3 million of outstanding contingent consideration related to acquisitions completed during or subsequent to fiscal 2010. Of this amount, $81.8 million was estimated as the fair value and accrued on our consolidated balance sheet. The following table summarizes the changes in the carrying value of estimated contingent earn-out liabilities:
		Fiscal Year Ended
		September 29,			September 30,			October 2,
		2013			2012			2011
		(in thousands)
	Beginning balance (at fair value)	$	51,539		$	75,159		$	20,504
	Estimated earn-out liabilities for acquisitions during the fiscal year		75,253			18,981			55,622
	Earn-out liabilities for acquisitions completed prior to fiscal 2010.		250			9,974			21,978
	Increases due to re-measurement of fair value reported in interest expense		2,433			1,374			1,612
	Net decreases due to re-measurement of fair value reported as gains in operating income		(9,560	)		(19,246	)		(1,755	)
	Foreign exchange impact		(2,480	)		3,027			(743	)
	Earn-out payments:
	Reported as cash used in operating activities		(695	)		(601	)		–
	Reported as cash used in investing activities		(1,279	)		(11,773	)		(22,059	)
	Reported as cash used in financing activities		(33,672	)		(18,055	)		–
	Settlement of receivables due from sellers		–			(7,301	)		–
	Ending balance (at fair value)	$	81,789		$	51,539		$	75,159

Goodwill_and_Intangible_Assets

Goodwill and Intangible Assets	12 Months Ended
	Sep. 29, 2013
Goodwill and Intangible Assets	'
Goodwill and Intangible Assets	'
	6.           Goodwill and Intangible Assets
	The following table summarizes the changes in the carrying value of goodwill:
		ECS			TSS			EAS			RCM			Total
		(in thousands)
	Balance at October 2, 2011	$	405,678		$	82,091		$	17,710		$	63,935		$	569,414
	Inter-segment transfer (1):
	Fiscal 2012 realignment		(29,338	)		45,435			–			(16,097	)		–
	Fiscal 2013 realignment		2,605			15,105			(17,710	)		–			–
	Goodwill additions		15,367			31,236			–			1,945			48,548
	Foreign exchange impact		18,910			–			–			–			18,910
	Goodwill impairment		(914	)		–			–			–			(914	)
	Balance at September 30, 2012		412,308			173,867			–			49,783			635,958
	Goodwill additions		14,364			3,594			–			145,064			163,022
	Foreign exchange impact		(16,464	)		118			–			(3,242	)		(19,588	)
	Goodwill impairment		(56,600	)		–			–			–			(56,600	)
	Balance at September 29, 2013	$	353,608		$	177,579		$	–		$	191,605		$	722,792
	-1
	Prior-year amounts have been reclassified due to realignment of certain operating activities in our reportable segments in fiscal 2012 and fiscal 2013. See Note 18, "Reportable Segments" for further information.
	Goodwill additions are primarily attributable to acquisitions described in Note 5, "Mergers and Acquisitions" for the respective fiscal years. Additionally, fiscal 2012 goodwill additions include earn-out payments associated with an acquisition consummated prior to fiscal 2010, which was accounted for as an increase to goodwill under previous accounting rules. Substantially all of the goodwill additions are not deductible for income tax purposes. Foreign exchange impact relates to our foreign subsidiaries with functional currencies that are different than our reporting currency. The gross amounts of goodwill, excluding accumulated impairment, for ECS were $413.2 million and $411.1 million for fiscal 2012 and 2013 year-ends, respectively.
	During the third quarter of fiscal 2013, certain of our reporting units experienced declines in their actual and projected financial performance. In Eastern Canada, poor economic conditions, including budget deficits, reduced customer spending, and an on-going government investigation into political corruption in Quebec slowed procurements and business activity in that region. In addition, our work for mining customers continued to slow at a faster pace than previously anticipated due to reduced demand and significant declines in prices for certain metals. To a lesser extent, we also experienced reduced performance from reporting units with a concentration of work for certain agencies of the U.S. federal government as a result of customer budgetary constraints. As a result of these factors, during the third quarter of fiscal 2013, we performed an interim goodwill impairment test for three reporting units in our ECS segment, as follows:
	•
	Tetra Tech Canada ("TTC"), with operations primarily in Eastern Canada, particularly Quebec;
	•
	Global Mining Practice ("GMP"), with operations primarily in the U.S., Canada, Australia and South America; and
	•
	Advanced Management Technology, Inc. ("AMT"), a U.S. federal government contractor primarily doing business with the FAA.
	We performed the first step of the impairment test for each of these reporting units during the third quarter of fiscal 2013, and in each case determined that the carrying value of the reporting unit exceeded its fair value indicating potential goodwill impairment. The significant change to the assumptions used in the interim test in the third quarter of fiscal 2013 compared to the last annual impairment test as of July 2, 2012 was the projected revenue, operating income and cash flows for each reporting unit tested.
	We performed the second step of the goodwill impairment test to measure the amount of the impairment loss, if any, of the applicable reporting units. The second step of the test requires the allocation of the reporting unit's fair value to its assets and liabilities, including any unrecognized intangible assets, in a hypothetical analysis that calculates the implied fair value of goodwill as if the reporting unit was being acquired in a business combination. If the implied fair value of goodwill is less than the carrying value, the difference is recorded as an impairment loss. Based on the results of the step two analyses, we recorded a $56.6 million, or $48.1 million, net of tax, goodwill impairment charge in the third quarter of fiscal 2013 related to the TTC, GMP and AMT reporting units. The carrying amounts of these reporting units, including goodwill were as follows:
		June 30, 2013
		TTC			GMP			AMT
		(in thousands)
	Carrying value before impairment	$	245,634		$	116,184		$	56,474
	Goodwill impairment		(27,900	)		(11,900	)		(16,800	)
	Carrying value after impairment	$	217,734		$	104,284		$	39,674
	As of June 30, 2013, the goodwill amounts after the impairment charges for the TTC, GMP and AMT reporting units were $111.1 million, $72.3 million and $32.6 million, respectively.
	The gross amount and accumulated amortization of our acquired identifiable intangible assets with finite useful lives included in "Intangible assets – net" on the consolidated balance sheets, were as follows:
		Fiscal Year Ended
		September 29, 2013									September 30, 2012
		Weighted-			Gross			Accumulated			Gross			Accumulated
		Average			Amount			Amortization			Amount			Amortization
		Remaining
		Life
		(in years)
		($ in thousands)
	Non-compete agreements		2.6		$	6,160		$	(5,247	)	$	5,467		$	(4,685	)
	Client relations		4.5			128,839			(49,189	)		99,096			(31,477	)
	Backlog		0.4			68,968			(64,675	)		59,931			(55,908	)
	Technology and trade names		2.9			4,204			(2,131	)		3,034			(1,227	)
	Total				$	208,171		$	(121,242	)	$	167,528		$	(93,297	)
	In fiscal 2013, the increases in gross amounts were attributable to the fiscal 2013 acquisitions described in Note 5, "Mergers and Acquisitions" and, to a lesser extent, foreign currency translation adjustments. Amortization expense for these intangible assets for fiscal 2013, 2012 and 2011 was $32.4 million, $29.6 million and $28.0 million, respectively. Estimated amortization expense for the succeeding five years and beyond is as follows:
		Amount
		(in thousands)
	2014	$	26,815
	2015		19,328
	2016		15,875
	2017		13,626
	2018		6,333
	Beyond		4,952
	Total	$	86,929

Property_and_Equipment

Property and Equipment	12 Months Ended
	Sep. 29, 2013
Property and Equipment	'
Property and Equipment	'
	7.           Property and Equipment
	The property and equipment consisted of the following:
		Fiscal Year Ended
		September 29,			September 30,
		2013			2012
		(in thousands)
	Land and buildings	$	5,565		$	5,537
	Equipment, furniture and fixtures		210,172			177,710
	Leasehold improvements		26,429			26,180
	Total property and equipment		242,166			209,427
	Accumulated depreciation		(154,140	)		(135,118	)
	Property and equipment, net	$	88,026		$	74,309
	The depreciation expense related to property and equipment, including assets under capital leases, was $29.5 million, $26.7 million and $27.1 million for fiscal 2013, 2012 and 2011, respectively.
	In fiscal 2012, one of our properties met the held for sale classification criteria at fiscal 2012 year-end. This property consists of land and a building at a net book value of $5.8 million. We estimated the fair value of this property using market values for similar properties, and this is considered a Level 3 measurement as defined in FASB's guidance on "Fair Value Measurements and Disclosures." After adjustment to fair value, the $2.4 million carrying value of this property was reclassified to "Prepaid expenses and other current assets" in the consolidated balance sheet at September 30, 2012. Additionally, we recorded the related non-cash impairment charge of $3.4 million in our corporate "Selling, general and administrative expenses" in the consolidated statement of operations for fiscal 2012.
	In connection with exit activities related to vacating leased facilities, we recorded a loss of $7.2 million in the second half of fiscal 2013. The loss consisted of an accrued liability of $4.8 million for estimated contract termination costs associated with the long-term non-cancelable leases of those facilities, reduced by $0.3 million of write-offs of prorated portions of existing deferred items previously recognized in connection with the leases, and $2.7 million in net write-offs of fixed assets, primarily leasehold improvements, furniture and fixtures, that were no longer in use after vacating the facilities. The loss is recorded in other costs of revenue on the consolidated statements of operations (see Note 10, "Leases" for further information).

Income_Taxes

Income Taxes	12 Months Ended
	Sep. 29, 2013
Income Taxes	'
Income Taxes	'
	8.           Income Taxes
	The income before income taxes, by geographic area, was as follows:
		Fiscal Year Ended
		September 29,			September 30,			October 2,
		2013			2012			2011
		(in thousands)
	Income (loss) before income taxes:
	United States	$	60,547		$	141,035		$	126,912
	Foreign		(48,015	)		19,761			13,580
	Total income before income taxes	$	12,532		$	160,796		$	140,492
	Income tax expense consisted of the following:
		Fiscal Year Ended
		September 29,			September 30,			October 2,
		2013			2012			2011
		(in thousands)
	Current:
	Federal	$	11,155		$	46,058		$	30,246
	State		2,705			6,949			5,948
	Foreign		11,646			8,569			9,596
	Total current income tax expense		25,506			61,576			45,790
	Deferred:
	Federal		(2,965	)		(200	)		6,755
	State		(637	)		(622	)		1,069
	Foreign		(7,866	)		(4,690	)		(6,104	)
	Total deferred income tax expense (benefit)		(11,468	)		(5,512	)		1,720
	Total income tax expense	$	14,038		$	56,064		$	47,510
	Total income tax expense was different from the amount computed by applying the U.S. federal statutory rate to pre-tax income as follows:
		Fiscal Year Ended
		September 29, 2013						September 30, 2012					October 2, 2011
		($ in thousands)
	Tax at federal statutory rate	$	4,386			35	%	$	56,278		35	%	$	49,172		35	%
	State taxes, net of federal benefit		1,316			10.5			4,932		3.1			4,376		3.1
	R&E credits		(6,622	)		(52.8	)		(360	)	(0.2	)		(1,689	)	(1.2	)
	Domestic production deduction		(828	)		(6.6	)		(774	)	(0.5	)		(770	)	(0.6	)
	Tax differential on foreign earnings		(4,263	)		(34.0	)		(4,444	)	(2.8	)		(4,140	)	(3.0	)
	Corrections of prior-year errors		3,255			26			–		–			–		-
	Goodwill and contingent consideration		11,288			90			(1,552	)	(1.0	)		–		-
	Stock compensation		443			3.5			80		0.1			301		0.2
	Valuation allowance		4,947			39.5			2,512		1.6			-		-
	Other		116			0.9			(608	)	(0.4	)		260		0.3
	Total income tax expense	$	14,038			112	%	$	56,064		34.9	%	$	47,510		33.8	%
	Our fiscal year 2013 effective tax rate was 112.0% compared to 34.9% for fiscal 2012. The higher effective tax rate resulted primarily from impairment charges and valuation allowance. We are currently under examination by the Internal Revenue Service for the fiscal years 2005 through 2009, and by the California Franchise Tax Board for fiscal years 2004 through 2005, with respect to R&E credits. We are also subject to various other state audits. With a few exceptions, we are no longer subject to U.S. federal, state and local, or non-U.S. income tax examinations for fiscal years before 2004.
	Temporary differences comprising the net deferred income tax liability shown on the accompanying consolidated balance sheets were as follows:
		Fiscal Year Ended
		September 29,			September 30,
		2013			2012
		(in thousands)
	Deferred Tax Asset:
	State taxes	$	452		$	975
	Reserves and contingent liabilities		5,883			4,689
	Allowance for doubtful accounts		7,345			2,039
	Accrued liabilities		14,425			13,298
	Stock-based compensation		10,778			10,980
	Loss carry-forwards		9,563			2,926
	Valuation allowance on loss carry-forwards		(7,459	)		(2,512	)
	Total deferred tax asset		40,987			32,395
	Deferred Tax Liability:
	Unbilled revenue		(47,281	)		(45,417	)
	Prepaid expense		(7,522	)		(2,251	)
	Intangibles		(24,933	)		(21,695	)
	Property and equipment		(9,946	)		(8,109	)
	Total deferred tax liability		(89,682	)		(77,472	)
	Net deferred tax liability	$	(48,695	)	$	(45,077	)
We have performed an assessment of positive and negative evidence regarding the realization of the deferred tax assets at September 29, 2013. This assessment included the evaluation of scheduled reversals of deferred tax liabilities, availability of carry-backs, and estimates of projected future taxable income. Although realization is not assured, based on our assessment, we have concluded that it is more likely than not that the assets will be realized except for the assets related to loss carry-forwards in foreign jurisdictions for which a valuation allowance of $7.5 million has been provided.
At September 29, 2013, undistributed earnings of our foreign subsidiaries, primarily in Canada, amounting to approximately $23.5 million are expected to be permanently reinvested. Accordingly, no provision for U.S. income taxes or foreign withholding taxes has been made. Upon distribution of those earnings, we would be subject to U.S. income taxes and foreign withholding taxes. Determination of the amount of unrecognized deferred U.S. income tax liability is not practicable; however, the potential foreign tax credit associated with the deferred income would be available to partially reduce the resulting U.S. tax liabilities.
At September 29, 2013, we had $25.9 million of unrecognized tax benefits. Included in the balance of unrecognized tax benefits at the end of fiscal year 2013 were $24.1 million of tax benefits that, if recognized, would affect our effective tax rate. It is not expected that there will be a significant change in the unrecognized tax benefits in the next 12 months. A reconciliation of the beginning and ending amount of unrecognized tax benefits is as follows:
	Fiscal Year Ended
	September 29,			September 30,			October 2,
	2013			2012			2011
	(in thousands)
Beginning balance	$	24,092		$	25,940		$	21,806
Additions for current year tax positions		2,661			6,273			8,007
Additions for prior year tax positions		4,951			19			2,554
Reductions for prior year tax positions		(5,818	)		(8,072	)		(6,315	)
Settlements		–			(68	)		(112	)
Ending balance	$	25,886		$	24,092		$	25,940
We recognize potential interest and penalties related to unrecognized tax benefits in income tax expense. The amount of interest expense (net of interest income) accrued at September 29, 2013 and September 30, 2012, was $2.3 million and $3.1 million, respectively.

LongTerm_Debt

Long-Term Debt	12 Months Ended
	Sep. 29, 2013
Long-Term Debt	'
Long-Term Debt	'
	9.           Long-Term Debt
	Long-term debt consisted of the following:
		Fiscal Year Ended
		September 29,			September 30,
		2013			2012
		(in thousands)
	Credit facilities	$	205,000		$	79,233
	Other		2,749			3,845
	Total long-term debt		207,749			83,078
	Less: Current portion of long-term debt		(4,311	)		(2,031	)
	Long-term debt, less current portion	$	203,438		$	81,047
	At September 30, 2012, we had a credit agreement that provided for a $460 million five-year revolving credit facility that matured in March 2016. On May 7, 2013, we entered into the Amended Credit Agreement and refinanced the indebtedness under the prior credit agreement. The Amended Credit Agreement is a $665 million senior secured, five-year facility that provides for a $205 million Term Loan Facility and a $460 million Revolving Credit Facility. The Amended Credit Agreement allows us to, among other things, finance certain permitted open market repurchases of our common stock, permitted acquisitions, and cash dividends and distributions. The Revolving Credit Facility includes a $200 million sublimit for the issuance of standby letters of credit, a $20 million sublimit for swingline loans, and a $150 million sublimit for multicurrency borrowings and letters of credit. Borrowings under the Amended Credit Agreement are collateralized by our accounts receivable, the stock of certain of our subsidiaries, and intercompany loans. The Amended Credit Agreement expires on May 7, 2018, or earlier at our discretion upon payment in full of loans and other obligations. We had borrowings outstanding under the Amended Credit Agreement at September 29, 2013 of $205.0 million, entirely under the Term Loan Facility, at a weighted-average interest rate of 1.56% per annum. Borrowings during fiscal 2013 under our credit facilities were at a weighted-average interest rate of 1.84% per annum. At September 29, 2013, there was $12.4 million outstanding in standby letters of credit under the Amended Credit Agreement. At September 29, 2013, we had $447.6 million of available credit under the Revolving Credit Facility, of which $265.5 million could be borrowed without a violation of our debt covenants.
	The Term Loan Facility is subject to quarterly amortization of principal, with no principal payment due in year 1, $10.3 million payable in both years 2 and 3, and $15.4 million payable in both years 4 and 5, respectively, with any unpaid balance due at maturity. The Term Loan may be prepaid at any time without penalty. We may borrow on the Revolving Credit Facility, at our option, at either (a) a Eurocurrency rate plus a margin that ranges from 1.15% to 2.00% per annum, or (b) a base rate for loans in U.S. dollars (the highest of the U.S. federal funds rate plus 0.50% per annum, the bank's prime rate or the Eurocurrency rate plus 1.00%) plus a margin that ranges from 0.15% to 1.00% per annum. In each case, the applicable margin is based on our Consolidated Leverage Ratio, calculated quarterly. The Term Loan Facility is subject to the same interest rate provisions.
	The Amended Credit Agreement contains certain affirmative and restrictive covenants, and customary events of default. The financial covenants provide for a maximum Consolidated Leverage Ratio of 2.50 to 1.00 and a minimum Consolidated Fixed Charge Coverage Ratio of 1.25 to 1.00. Our obligations under the Amended Credit Agreement are guaranteed by certain of our subsidiaries and are secured by first priority liens on (i) the equity interests of certain of our subsidiaries, including those subsidiaries that are guarantors or borrowers under the Amended Credit Agreement, and (ii) our accounts receivable, general intangibles and intercompany loans, and those of our subsidiaries that are guarantors or borrowers. As of September 29, 2013, we met all compliance requirements of these covenants.
	On September 27, 2013, we entered into Amendment No. 1 to the Amended Credit Agreement to amend the definition of "Consolidated EBITDA" for purposes of the financial covenants contained in the Amended Credit Agreement to add back to Consolidated Net Income for the fiscal quarters ending September 29, 2013, December 29, 2013 and March 30, 2014 (i) up to $34 million in non-recurring charges incurred during the fiscal quarter ended June 30, 2013 in connection with corporate restructurings and (ii) up to $36 million in non-cash charges incurred during the fiscal quarter ended June 30, 2013 in connection with the Four Programs referenced in our Form 8-Ks, filed with the SEC on June 18, 2013 and August 7, 2013, and Form 10-Q for the fiscal quarter ended June 30, 2013. Amendment No. 1 also provides that Consolidated EBITDA will be calculated without giving effect to the add-backs referenced above for purposes of determining our applicable margin in effect at any time.
	In fiscal 2013, other debt includes capital leases of $1.8 million, property and equipment loans of $0.1 million, and a bank overdraft facility of $0.9 million at one of our foreign affiliates. In fiscal 2012, other debt includes capital leases of $2.8 million, property and equipment loans of $0.5 million, and a bank overdraft facility of $0.5 million at one of our foreign affiliates.
	We have three letter of credit agreements with three banks to issue up to $40 million in standby letters of credit. The amount of standby letters of credit outstanding under these facilities at September 29, 2013 was $6.5 million, of which $6.4 million was issued in currencies other than the U.S. dollar.
	The following table presents scheduled maturities of our long-term debt:
		Amount
		(in thousands)
	2014	$	4,311
	2015		10,868
	2016		15,753
	2017		15,379
	2018		161,438
	Total	$	207,749

Leases

Leases	12 Months Ended
	Sep. 29, 2013
Leases	'
Leases	'
	10.         Leases
	We lease office and field equipment, vehicles and buildings under various operating leases. In fiscal 2013, 2012 and 2011, we recognized $80.8 million, $76.6 million and $71.9 million of expense associated with operating leases, respectively. Amounts payable under non-cancelable operating and capital lease commitments are as follows during the following fiscal years:
		Operating			Capital
		(in thousands)
	2014	$	71,293		$	874
	2015		54,169			648
	2016		35,265			384
	2017		23,898			5
	2018		14,771			–
	Beyond		24,628			–
	Total	$	224,024			1,911
	Less: Amounts representing interest					(132	)
	Net present value				$	1,779
	We vacated certain facilities under long-term non-cancelable leases and recorded contract termination costs of $4.5 million in fiscal 2013, $1.3 million in fiscal 2012 and $1.3 million in fiscal 2011. These amounts were initially measured at the fair value of the portion of the lease payments associated with the vacated facilities, reduced by estimated sublease rentals, less the write off of a prorated portion of existing deferred items previously recognized on these leases. We expect the remaining lease payments to be paid through the various lease expiration dates that continue until 2019.
	We initially measured the lease contract termination liability at the fair value of the prorated portion of the lease payments associated with the vacated facilities, reduced by estimated sublease rentals and other costs. If the actual timing and potential termination costs or realization of sublease income differ from our estimates, the resulting liabilities could vary from recorded amounts. These liabilities are reviewed periodically and adjusted when necessary. We expect the remaining lease payments to be paid through the various lease expiration dates that continue until 2021. The following is a reconciliation of the beginning and ending balances of these liabilities related to lease contract termination costs:
		ECS			TSS			Total
		(in thousands)
	Balance at September 30, 2012	$	–		$	2,940		$	2,940
	Costs incurred and charged to expense		3,744			1,055			4,799
	Adjustments (1)		(34	)		(1,432	)		(1,466	)
	Balance at September 29, 2013	$	3,710		$	2,563		$	6,273
	-1
	Adjustments of the actual timing and potential termination costs or realization of sublease income.

Stockholders_Equity_and_Stock_

Stockholders' Equity and Stock Compensation Plans	12 Months Ended
	Sep. 29, 2013
Stockholders' Equity and Stock Compensation Plans	'
Stockholders' Equity and Stock Compensation Plans	'
	11.         Stockholders' Equity and Stock Compensation Plans
	At September 29, 2013, we had the following stock-based compensation plans:
	•
	2003 Outside Director Stock Option Plan.  Non-employee directors may be granted options to purchase an aggregate of up to 400,000 shares of our common stock at prices not less than 100% of the market value on the date of grant. Exercise prices of all options granted were at the market value on the date of grant. These options vest and become exercisable on the first anniversary of the grant date if the director has not ceased to be a director prior to such date, and expire no later than ten years from the grant date.
	•
	2005 Equity Incentive Plan ("EIP").  Key employees and non-employee directors may be granted equity awards, including stock options, restricted stock and RSUs, with respect to an aggregate of 6,086,216 shares of our common stock. Options granted before March 6, 2006 vest at 25% on the first anniversary of the grant date, and the balance vests monthly thereafter, such that these options become fully vested no later than four years from the date of grant. These options expire no later than ten years from the date of grant. Options granted on and after March 6, 2006 vest at 25% on each anniversary of the grant date. These options expire no later than eight years from the grant date. RSUs granted to date vest at 25% on each anniversary of the grant date.
	In accordance with our Executive Compensation Policy, our Compensation Committee has awarded restricted stock to executive officers and non-employee directors under the EIP. Restricted stock grants generally vest over a minimum three-year period, and may be performance-based, determined by EPS growth, or service-based.
	•
	Employee Stock Purchase Plan ("ESPP").  Purchase rights to purchase common stock are granted to our eligible full and part-time employees, and shares of common stock are issued upon exercise of the purchase rights. An aggregate of 2,373,290 shares may be issued pursuant to such exercise. The maximum amount that an employee can contribute during a purchase right period is $5,000. The exercise price of a purchase right is the lesser of 100% of the fair market value of a share of common stock on the first day of the purchase right period or 85% of the fair market value on the last day of the purchase right period (December 15, or the business day preceding December 15 if December 15 is not a business day).
	The stock-based compensation and related income tax benefits were as follows:
		Fiscal Year Ended
		September 29,			September 30,			October 2,
		2013			2012			2011
		(in thousands)
	Total stock-based compensation	$	8,775		$	10,839		$	10,582
	Income tax benefit related to stock-based compensation		(3,048	)		(4,288	)		(3,804	)
	Stock-based compensation, net of tax benefit	$	5,727		$	6,551		$	6,778
	Stock Options
	Stock option activity for the fiscal year ended September 29, 2013 was as follows:
		Number of			Weighted-			Weighted-			Aggregate
		Options			Average			Average			Intrinsic Value
		(in thousands)			Exercise Price			Remaining			(in thousands)
					per Share			Contractual Term
								(in years)
	Outstanding on September 30, 2012		4,876		$	21.5
	Granted		287			24.24
	Exercised		(793	)		20.18
	Cancelled		(106	)		23.72
	Outstanding at September 29, 2013		4,264		$	21.88			3.7		$	17,502
	Vested or expected to vest at September 29, 2013		4,229		$	21.9			3.7		$	17,268
	Exercisable on September 29, 2013		3,079		$	21.16			5.4		$	14,829
	The aggregate intrinsic value in the table above represents the total intrinsic value (the difference between our closing stock price on the last trading day of fiscal 2012 and the exercise price, times the number of shares) that would have been received by the in-the-money option holders if they had exercised their options on September 29, 2013. This amount will change based on the fair market value of our stock. At September 29, 2013, we expect to recognize $6.3 million of unrecognized compensation cost related to stock option grants over a weighted-average period of 1.7 years. At September 29, 2013, there were approximately 3.4 million options available for future awards.
	The weighted-average fair value of stock options granted during fiscal 2013, 2012 and 2011 was $8.74, $8.37 and $9.08, respectively. The aggregate intrinsic value of options exercised during fiscal 2013, 2012 and 2011 was $6.4 million, $6.1 million and $2.3 million, respectively.
	The fair value of our stock options was estimated on the date of grant using the Black-Scholes option pricing model. The following assumptions were used in the calculation:
		Fiscal Year Ended
		September 29,			September 30,			October 2,
		2013			2012			2011
	Dividend yield		–			–			–
	Expected stock price volatility		41.7% - 42.2%			41.9% - 44.0%			41.8 - 42.7%
	Risk-free rate of return, annual		0.6% - 1.3%			0.7% - 1.1%			1.3 - 2.1%
	For purposes of the Black-Scholes model, forfeitures were estimated based on historical experience. For the fiscal 2013, 2012 and 2011 year-ends, we based our expected stock price volatility on historical volatility behavior and current implied volatility behavior. Our risk-free rate of return was based on constant maturity rates provided by the U.S. Treasury. The expected life was based on historical experience.
	Net cash proceeds from the exercise of stock options were $16.0 million, $18.2 million and $8.4 million for fiscal 2013, 2012 and 2011, respectively. Our policy is to issue shares from our authorized shares upon the exercise of stock options. The actual income tax benefit realized from exercises of nonqualified stock options and disqualifying dispositions of qualified options for fiscal 2013, 2012 and 2011 was $3.7 million, $3.2 million and $1.4 million, respectively.
	Restricted Stock and RSUs
	In fiscal 2013, 2012 and 2011, we awarded 108,350 shares, 105,567 shares and 94,606 shares, respectively, of restricted stock to certain of our executive officers and non-employee directors. Of the aggregate total of 308,523 awards, vesting as to 10,000 shares is time-based, and is dependent on the officer's continued employment, or the non-employee director's continued service, with us, but otherwise vest over a three-year period. As to the remaining 298,523 shares, vesting is performance-based, such that the percentage of awarded shares that ultimately vests, from 0% to 140%, is dependent on fiscal year earnings per share growth rates for the three fiscal years that end after the award date. In fiscal 2013, 2012 and 2011, an additional 4,947 shares, 5,305 shares and 8,356 shares of restricted stock, respectively, were awarded for performance-based adjustments in excess of 100% vesting. Restricted stock forfeitures result from employment terminations prior to vesting, and from performance-based vesting of less than 100%. Forfeited shares return to the pool of authorized shares available for award. As of September 29, 2013, there were 938,114 shares available for future awards of restricted stock.
	Restricted stock activity for the fiscal year ended September 29, 2013 was as follows:
		Number of			Weighted-
		Shares			Average Grant
		(in thousands)			Date Fair
					Value
	Nonvested balance at September 30, 2012		190		$	23.08
	Granted		113			24.23
	Vested		(96	)		23.45
	Forfeited		(4	)		22.85
	Nonvested balance at September 29, 2013		203		$	23.55
	Vested or expected to vest at September 29, 2013		203		$	23.55
	The fair value of the total compensation cost of each restricted stock award was determined at the date of grant using the market price of the underlying common stock as of the date of grant. For performance-based awards, our expected performance is reviewed to estimate the percentage of shares that will vest. The total compensation cost of the awards is then amortized over their applicable vesting period on a straight-line basis.
	In fiscal 2013, we also awarded 226,655 RSUs to our employees at the fair value of $24.32 per share on the award date. All of the RSUs have time-based vesting over a four-year period. At September 29, 2013, there were 333,140 RSUs outstanding. RSU forfeitures result from employment terminations prior to vesting. Forfeited shares return to the pool of authorized shares available for award.
	In the first quarter of fiscal 2012, we also awarded 181,348 RSUs to our employees at the fair value of $22.53 per share on the award date. All of the RSUs have time-based vesting over a four-year period.
	The stock-based compensation expense related to restricted stock and RSUs for fiscal years 2013, 2012 and 2011 was $2.2 million, $3.0 million and $1.7 million, respectively, and was included in the total stock-based compensation expense. At September 29, 2013, there was $7.1 million of unrecognized compensation costs related to restricted stock and RSUs that will be substantially recognized by the end of fiscal 2017.
	RSU activity for the fiscal year ended September 29, 2013 was as follows:
		Number of			Weighted-
		Shares			Average Grant
		(in thousands)			Date Fair
		Value
Nonvested balance at September 30, 2012		172		$	22.53
Granted		227			24.32
Vested		(43	)		22.53
Forfeited		(23	)		23.27
Nonvested balance at September 29, 2013		333		$	23.7
ESPP
The following table summarizes shares purchased, weighted-average purchase price, cash received and the aggregate intrinsic value for shares purchased under the ESPP:
	Fiscal Year Ended
	September 29,			September 30,			October 2,
	2013			2012			2011
	(in thousands, except for purchase price)
Shares purchased		253			289			246
Weighted-average purchase price	$	21.96		$	18.35		$	21.3
Cash received from exercise of purchase rights	$	5,551		$	5,300		$	5,249
Aggregate intrinsic value	$	1,140		$	935		$	926
The grant date fair value of each award granted under the ESPP was estimated using the Black-Scholes option pricing model with the following assumptions:
	Fiscal Year Ended
	September 29,			September 30,			October 2,
	2013			2012			2011
Dividend yield		-			-			-
Expected stock price volatility		27.1	%		34.7	%		38	%
Risk-free rate of return, annual		0.1	%		0.1	%		0.3	%
Expected life (in years)		1			1			1
For fiscal 2013, 2012 and 2011, we based our expected stock price volatility on historical volatility behavior and current implied volatility behavior. The risk-free rate of return was based on constant maturity rates provided by the U.S. Treasury. The expected life was based on the ESPP terms and conditions.
Included in stock-based compensation expense for fiscal 2013, 2012 and 2011 was $0.8 million, $0.9 million and $1.0 million, respectively, related to the ESPP. The unrecognized stock-based compensation costs for awards granted under the ESPP at September 29, 2013 and September 30, 2012, were $0.2 million and $0.2 million, respectively. At September 29, 2013, ESPP participants had accumulated $3.1 million to purchase our common stock.

Retirement_Plans

Retirement Plans	12 Months Ended
	Sep. 29, 2013
Retirement Plans	'
Retirement Plans	'
	12.         Retirement Plans
	We have established defined contribution plans including 401(k) plans. Generally, employees are eligible to participate in the defined contribution plans upon completion of one year of service and in the 401(k) plans upon commencement of employment. For fiscal 2013, 2012 and 2011, employer contributions to the plans were $9.5 million, $14.7 million and $14.1 million, respectively.
	We have established a non-qualified deferred compensation plan for certain key employees and non-employee directors. Eligible employees and non-employee directors may elect to defer the receipt of salary, incentive payments, restricted stock and RSU awards, and non-employee director fees, which are generally invested by us in individual variable life insurance contracts we own that are designed to informally fund savings plans of this nature. At September 29, 2013 and September 30, 2012, the consolidated balance sheets reflect assets of $17.2 million and $13.4 million, respectively, related to the deferred compensation plan in "Other long-term assets," and liabilities of $16.1 million and $12.9 million, respectively, related to the deferred compensation plan in "Other long-term liabilities."

Earnings_Per_Share

Earnings Per Share	12 Months Ended
	Sep. 29, 2013
Earnings Per Share	'
Earnings Per Share	'
	13.         Earnings Per Share
	The following table sets forth the number of weighted-average shares used to compute basic and diluted EPS:
		Fiscal Year Ended
		September 29,			September 30,		October 2,
		2013			2012		2011
		(in thousands, except per share data)
	Net income (loss) attributable to Tetra Tech	$	(2,141	)	$	104,380	$	90,039
	Weighted-average common shares outstanding – basic		64,544			63,217		62,053
	Effect of diluted stock options and unvested restricted stock		–			717		722
	Weighted-average common stock outstanding – diluted		64,544			63,934		62,775
	Net income (loss) attributable to Tetra Tech per share:
	Basic	$	(0.03	)	$	1.65	$	1.45
	Diluted	$	(0.03	)	$	1.63	$	1.43
	The computation of diluted loss per share for fiscal 2013 excludes 0.5 million of potential common shares due to their anti-dilutive effect. For fiscal 2012 and 2011, 1.9 million and 2.6 million options were excluded from the calculation of dilutive potential common shares, respectively. These options were not included in the computation of dilutive potential common shares because the assumed proceeds per share exceeded the average market price per share for that period. Therefore, their inclusion would have been anti-dilutive.

Derivative_Financial_Instrumen

Derivative Financial Instruments	12 Months Ended
	Sep. 29, 2013
Derivative Financial Instruments	'
Derivative Financial Instruments	'
	14.         Derivative Financial Instruments
	We use certain interest rate derivative contracts to hedge interest rate exposures on our variable rate debt. We enter into foreign currency derivative contracts with financial institutions to reduce the risk that cash flows and earnings will be adversely affected by foreign currency exchange rate fluctuations. Our hedging program is not designated for trading or speculative purposes.
	We recognize derivative instruments as either assets or liabilities on the accompanying consolidated balance sheets at fair value. We record changes in the fair value (i.e., gains or losses) of the derivatives that have been designated as accounting hedges in our consolidated balance sheets as accumulated other comprehensive income.
	In fiscal 2009, we entered into an intercompany promissory note with a wholly-owned Canadian subsidiary in connection with the acquisition of Wardrop Engineering, Inc. The intercompany note receivable is denominated in CAD and has a fixed rate of interest payable in CAD. In the second quarter of fiscal 2010, we entered into a forward contract for CAD $4.2 million (equivalent to U.S. $3.9 million at the date of inception) that matured on January 28, 2013. In the third quarter of fiscal 2011, we entered into a forward contract for CAD $4.2 million (equivalent to U.S. $4.2 million at the date of inception) with a maturity date of January 27, 2014. Our objective was to eliminate variability of our cash flows on the amount of interest income we receive on the promissory note from changes in foreign currency exchange rates. These contracts were designated as cash flow hedges. Accordingly, changes in the fair value of the contracts were recorded in "Other comprehensive income". In the second quarter of fiscal 2013, we settled one of the foreign currency forward contracts for U.S. $3.9 million and terminated the remaining forward contract. As a result, we recognized immaterial gains and losses in our consolidated statements of operations for fiscal 2013, 2012 and 2011.
	In fiscal 2013, we entered into three interest rate swap agreements that we have designated as cash flow hedges to fix the variable interest rates on a portion of borrowings under our Term Loan Facility. At September 29, 2013, the effective portion of our interest rate swap agreements designated as cash flow hedges before tax effect was $0.9 million, of which $0.9 million is expected to be reclassified from accumulated other comprehensive income to interest expense within the next 12 months.
	As of September 29, 2013, the notional principal, fixed rates and related expiration dates of our outstanding interest rate swap agreements are as follows:
	Notional Amount				Fixed			Expiration
	(in thousands)				Rate			Date
	$		51,250			1.36	%		May-18
			51,250			1.34	%		May-18
			51,250			1.35	%		May-18
	The fair values of our outstanding derivative instruments were as follows (in thousands):
				Fair Value of Derivative
				Instruments as of
		Balance Sheet Location		September 29,			September 30,
				2013			2012
	Derivatives designated as hedging instruments:
	Foreign currency forward contracts	Other current liabilities		$	–		$	348
	Interest rate swap agreements	Other current liabilities			987			–
	Total			$	987		$	348
	The impact of the effective portions of derivative instruments in cash flow hedging relationships on income and other comprehensive income from our foreign currency forward contracts and interest rate swap agreements was immaterial for the fiscal years ended September 30, 2012 and September 29, 2013. Additionally, there were no ineffective portions of derivative instruments. Accordingly, no amounts were excluded from effectiveness testing for our foreign currency forward contracts and interest rate swap agreements. We had no derivative instruments that were not designated as hedging instruments for fiscal 2013, 2012 and 2011.

Fair_Value_Measurements

Fair Value Measurements	12 Months Ended
	Sep. 29, 2013
Fair Value Measurements	'
Fair Value Measurements	'
	15.         Fair Value Measurements
	Derivative Instruments.    For additional information about our derivative financial instruments (see Note 2, "Basis of Presentation and Preparation" and Note 14 "Derivative Financial Instruments" for more information).
	Contingent Consideration.    We measure our contingent earn-out liabilities at fair value on a recurring basis (see Note 2, "Basis of Presentation and Preparation" and Note 5, "Mergers and Acquisitions" for further information).
	Debt.    The fair value of long-term debt was determined using the present value of future cash flows based on the borrowing rates currently available for debt with similar terms and maturities (Level 2 measurement, as described in "Critical Accounting Policies and Estimates"). The carrying value of our long-term debt approximated fair value at September 29, 2013 and September 30, 2012. For fiscal 2013, we had a net borrowing of $205.0 million under our amended credit agreement to fund our business acquisitions, working capital needs and contingent earn-outs (see Note 9, "Long-Term Debt" for more information).

Joint_Ventures

Joint Ventures	12 Months Ended
	Sep. 29, 2013
Joint Ventures	'
Joint Ventures	'
	16.         Joint Ventures
	Consolidated Joint Ventures
	The aggregate revenue of the consolidated joint ventures was $15.6 million, $19.3 million and $74.3 million for fiscal 2013, 2012 and 2011, respectively. The assets and liabilities of these consolidated joint ventures were immaterial at fiscal 2013, 2012 and 2011 year-ends. These assets are restricted for use only by those joint ventures and are not available for our general operations. Restricted cash and cash equivalents at September 29, 2013 and September 30, 2012 were $1.2 million and $1.6 million, respectively.
	Unconsolidated Joint Ventures
	We account for our unconsolidated joint ventures using the equity method of accounting. Under this method, we recognize our proportionate share of the net earnings of these joint ventures within "Other costs of revenue" in our consolidated statements of operations. For fiscal 2013, 2012 and 2011, we reported $3.5 million, $2.9 million and $4.9 million of equity in earnings of unconsolidated joint ventures, respectively. Our maximum exposure to loss as a result of our investments in unconsolidated variable interest entities is typically limited to the aggregate of the carrying value of the investment. Future funding commitments for the unconsolidated joint ventures are immaterial. The unconsolidated joint ventures are, individually and in aggregate, immaterial to our consolidated financial statements.
	The aggregate carrying values of the assets and liabilities of the unconsolidated joint ventures were $24.0 million and $21.8 million, respectively, at September 29, 2013, and $19.0 million and $15.7 million, respectively, at September 30, 2012.

Commitments_and_Contingencies

Commitments and Contingencies	12 Months Ended
	Sep. 29, 2013
Commitments and Contingencies	'
Commitments and Contingencies	'
	17.         Commitments and Contingencies
	We are subject to certain claims and lawsuits typically filed against the engineering, consulting and construction profession, alleging primarily professional errors or omissions. We carry professional liability insurance, subject to certain deductibles and policy limits, against such claims. However, in some actions, parties are seeking damages that exceed our insurance coverage or for which we are not insured. While management does not believe that the resolution of these claims will have a material adverse effect, individually or in aggregate, on our financial position, results of operations or cash flows, management acknowledges the uncertainty surrounding the ultimate resolution of these matters.
	We acquired BPR, a Quebec-based engineering firm on October 4, 2010. Subsequently, we have been informed of the following with respect to pre-acquisition activities at BPR:
	On April 17, 2012, authorities in the province of Quebec, Canada charged two employees of BPR Triax, a subsidiary of BPR, and BPR Triax, under the Canadian Criminal Code with allegations of corruption. Discovery procedures associated with the charges are currently ongoing, and the legal process is expected to continue into fiscal 2014. We have conducted an internal investigation concerning this matter and, based on the results of our investigation, we believe these allegations are limited to activities at BPR Triax prior to our acquisition of BPR.
	During late March 2013, the then-president of BPR gave testimony to the Charbonneau Commission, which is investigating possible corruption in the engineering industry in Quebec. He stated that during 2007 and 2008, he and other former BPR shareholders paid personal funds to a political party official in exchange for the award of five government contracts. Further, prior to the testimony, we were not aware of the misconduct. We have accepted the resignation of BPR's former president, and are evaluating the impact of these pre-acquisition actions on our business and results of operations.
	During March 2013, following the resignation of BPR's former president, we learned that criminal charges had been filed against BPR and its former president in France. The charges relate to allegations that, in 2009, a BPR subsidiary had hired an employee of another firm to be CEO of that BPR subsidiary as a part of a corrupt scheme that allegedly damaged, among others, the employee's former employer. A trial in this matter is scheduled for May 2014.
	On April 19, 2013, a class action proceeding was filed in Montreal in which BPR, BPR's former president, and other Quebec-based engineering firms and individuals are named as defendants. The plaintiff class includes all individuals and entities that have paid real estate or municipal taxes to the city of Montreal. The allegations include participation in collusion to share contracts awarded by the City of Montreal, conspiracy to reduce competition and fix prices, payment of bribes to officials, making illegal political contributions, and bid rigging.
	On June 28, 2013, a purported class action lawsuit was filed against Tetra Tech and two of our officers in United States District Court for the Central District of California. The action was purportedly brought on behalf of purchasers of our publicly traded securities between May 3, 2012 and June 18, 2013. The complaint alleges generally that we and those officers violated Sections 10(b) and 20(a) of the Securities Exchange Act of 1934 and related rules because we allegedly failed to take unspecified, "necessary" charges to our accounts receivables and earnings during the class period. In addition, the complaint alleges that the financial guidance we offered during the class period was intentionally or recklessly false and misleading. The complaint alleges unspecified damages based on the decline in the market price of our shares following the issuance of revised guidance on June 18, 2013. On October 30, 2013, plaintiff filed an amended complaint for the same purported class period making essentially the same allegations. We believe the case is without merit and intend to defend it vigorously.
	The financial impact to us of the matters discussed above is unknown at this time.

Reportable_Segments

Reportable Segments	12 Months Ended
	Sep. 29, 2013
Reportable Segments	'
Reportable Segments	'
	18.         Reportable Segments
	In the first quarter of fiscal 2013, we implemented a reorganization of our operations to improve future growth and profitability. These activities included the consolidation and realignment of certain operating activities to improve organizational effectiveness and achieve efficiencies in our segment management. This reorganization included the elimination of the EAS segment. Operating activities previously reported in this segment were realigned to operations with similar client types, project types and financial metrics in the ECS and TSS segments. Segment results for the prior year have been revised to conform to the current-year presentation.
	Our reportable segments for fiscal 2013 were as follows:
	Engineering and Consulting Services.    ECS provides front-end science, consulting engineering and project management services in the areas of surface water management, water infrastructure, solid waste management, mining, geotechnical sciences, arctic engineering, industrial processes and oil sands, transportation and information technology.
	Technical Support Services.    TSS provides management consulting and engineering services and strategic direction in the areas of environmental assessments/hazardous waste management, climate change, international development, international reconstruction and stabilization, energy, oil and gas, technical government consulting, and building and facilities.
	Remediation and Construction Management.    RCM provides full-service support, including construction and construction management, to all of our client sectors, including the U.S. federal government in the United States and internationally, and commercial clients worldwide, in the areas of environmental remediation, infrastructure development, solid waste management, energy, and oil and gas.
	Management evaluates the performance of these reportable segments based upon their respective segment operating income before the effect of amortization expense related to acquisitions and other unallocated corporate expenses. We account for inter-segment sales and transfers as if the sales and transfers were to third parties; that is, by applying a negotiated fee onto the costs of the services performed. All significant intercompany balances and transactions are eliminated in consolidation.
	The following tables set forth summarized financial information concerning our reportable segments:
	Reportable Segments
		Fiscal Year Ended
		September 29,			September 30,			October 2,
		2013			2012			2011
		(in thousands)
	Revenue
	ECS	$	1,035,983		$	1,155,256		$	1,110,060
	TSS		932,375			1,020,779			995,249
	RCM		725,689			621,957			604,651
	Elimination of inter-segment revenue		(80,292	)		(86,917	)		(136,816	)
	Total revenue	$	2,613,755		$	2,711,075		$	2,573,144
	Operating Income
	ECS	$	44,598		$	96,220		$	99,868
	TSS		71,842			71,767			69,977
	RCM		(6,706	)		22,374			13,183
	Corporate (1)		(89,516	)		(23,994	)		(36,606	)
	Total operating income	$	20,218		$	166,367		$	146,422
	Depreciation
	ECS	$	10,494		$	10,126		$	12,067
	TSS		2,839			3,227			2,028
	RCM		13,160			10,233			10,101
	Corporate		3,055			3,065			2,941
	Total depreciation	$	29,548		$	26,651		$	27,137
	-1
	Includes goodwill impairment charge, amortization of intangibles, other costs and other income not allocable to segments. The goodwill impairment charge of $56.6 million for fiscal 2013 was recorded at Corporate. The intangible asset amortization expense for fiscal 2013, 2012 and 2011 was $32.4 million, $29.6 million and $28.0 million, respectively. Corporate results also included income for fair value adjustments to contingent consideration liabilities of $9.6 million, $19.2 million and $1.8 million for 2013, 2012 and 2011, respectively.
		September 29,			September 30,
		2013			2012
		(in thousands)
	Total Assets
	ECS	$	912,996		$	915,571
	TSS		673,864			638,405
	RCM		435,053			311,051
	Corporate (1)		(222,821	)		(193,997	)
	Total assets	$	1,799,092		$	1,671,030
	-1
	Corporate assets consist of intercompany eliminations and assets not allocated to segments including goodwill, intangible assets, deferred income taxes and certain other assets.
	Geographic Information
		Fiscal Year Ended
		September 29, 2013						September 30, 2012					October 2, 2011
		Revenue			Long-Lived			Revenue			Long-Lived		Revenue		Long-Lived
					Assets (2)						Assets (2)				Assets (2)
								(in thousands)
	United States	$	1,915,780		$	110,313		$	2,046,700		$	100,958	$	1,976,452	$	102,316
	Foreign countries (1)		697,975			90,435			664,375			70,010		596,692		78,198
	-1
	Includes revenue generated from our foreign operations, primarily in Canada, and revenue generated from non-U.S. clients. Long-lived assets consist primarily of amounts from our Canadian operations.
	-2
	Excludes goodwill and intangible assets.
	Major Clients
	Other than the U.S. federal government, we had no single client that accounted for more than 10% of our revenue. All of our segments generated revenue from all client sectors.
	The following table presents our revenue by client sector:
		Fiscal Year Ended
		September 29,			September 30,			October 2,
		2013			2012			2011
		(in thousands)
Client Sector
International (1)	$	697,975		$	664,375		$	596,692
U.S commercial		693,677			718,457			577,782
U.S. federal government (2)		829,790			1,008,424			1,115,729
U.S. state and local government		392,313			319,819			282,941
Total	$	2,613,755		$	2,711,075		$	2,573,144
-1
Includes revenue generated from foreign operations, primarily in Canada, and revenue generated from non-U.S. clients.
-2
Includes revenue generated under U.S. federal government contracts performed outside the United States.

Quarterly_Financial_Informatio

Quarterly Financial Information-Unaudited	12 Months Ended
	Sep. 29, 2013
Quarterly Financial Information-Unaudited	'
Quarterly Financial Information-Unaudited	'
	19.         Quarterly Financial Information – Unaudited
	In the opinion of management, the following unaudited quarterly data for fiscal years ended September 29, 2013 and September 30, 2012 reflect all adjustments necessary for a fair statement of the results of operations.
	In the third quarter of fiscal 2013, we reported operating losses of $99.9 million, which included a non-cash goodwill impairment charge of $56.6 million. Additionally, we incurred project charges, including claims related to adverse developments on three programs in the RCM segment with U.S. federal and state and local government clients. We also recorded a project-related charge on a commercial development project in the TSS segment due to a change in client ownership and the related modification of plans for completion of the project. Collectively, project charges on these four programs reduced operating income by $35.5 million. Further, the weaker results in our Eastern Canada and global mining operations and the resulting charges to right-size these businesses caused a reduction of $28.2 million in operating income.
		First		Second		Third			Fourth
		Quarter		Quarter		Quarter			Quarter
		(in thousands, except per share data)
	Fiscal Year 2013
	Revenue	$	658,545	$	641,999	$	614,835		$	698,376
	Operating income (loss)		41,809		37,667		(99,884	)		40,626
	Net income (loss) attributable to Tetra Tech		26,224		24,820		(78,385	)		25,200
	Net income (loss) attributable to Tetra Tech per share (1):
	Basic	$	0.41	$	0.38	$	(1.21	)	$	0.39
	Diluted	$	0.41	$	0.38	$	(1.21	)	$	0.39
	Weighted-average common shares outstanding:
	Basic		63,864		64,551		64,832			64,272
	Diluted		64,608		65,472		64,832			64,853
	Fiscal Year 2012
	Revenue	$	682,627	$	624,345	$	684,698		$	719,405
	Operating income		36,093		35,543		46,261			48,470
	Net income attributable to Tetra Tech		22,610		22,284		29,054			30,432
	Earnings per share attributable to Tetra Tech (1):
	Basic	$	0.36	$	0.35	$	0.46		$	0.48
	Diluted	$	0.36	$	0.35	$	0.45		$	0.47
	Weighted-average common shares outstanding:
	Basic		62,433		63,072		63,387			63,623
	Diluted		63,068		63,817		64,179			64,396
	-1
	The sum of the quarterly EPS may not add up to the full-year EPS due to rounding.

SCHEDULE_IIVALUATION_AND_QUALI

SCHEDULE II-VALUATION AND QUALIFYING ACCOUNTS AND RESERVES	12 Months Ended
	Sep. 29, 2013
SCHEDULE II-VALUATION AND QUALIFYING ACCOUNTS AND RESERVES	'
SCHEDULE II-VALUATION AND QUALIFYING ACCOUNTS AND RESERVES	'
	TETRA TECH, INC.
	SCHEDULE II – VALUATION AND QUALIFYING ACCOUNTS AND RESERVES
	For the Fiscal Years Ended
	October 2, 2011, September 30, 2012 and September 29, 2013
	(in thousands)
		Balance at		Additions		Deductions (1)			Other (2)			Balance at
		Beginning of		(Charged to								End of Period
		Period		Costs, Expenses
				and Revenue)
	Allowance for doubtful accounts:
	Fiscal 2011	$	32,926	$	3,733	$	(6,478	)	$	2,063		$	32,244
	Fiscal 2012		32,244		4,768		(2,356	)		896			35,552
	Fiscal 2013		35,552		13,818		(4,452	)		(295	)		44,623
	Income tax valuation allowance:
	Fiscal 2011	$	5,526	$	–	$	-		$	(5,526	)	$	–
	Fiscal 2012		–		2,512		–			-			2,512
	Fiscal 2013		2,512		4,947		–			–			7,459
	-1
	Primarily represents uncollectible accounts written off, net of recoveries.
	-2
	Includes allowances from new business acquisitions, loss in foreign jurisdictions and currency adjustments, and represents valuation allowance adjustments related to net operating loss carry-forwards.

Basis_of_Presentation_and_Prep1

Basis of Presentation and Preparation (Policies)	12 Months Ended
	Sep. 29, 2013
Basis of Presentation and Preparation	'
Principles of Consolidation and Presentation	'
	Principles of Consolidation and Presentation.    The consolidated financial statements include our accounts and those of joint ventures of which we are the primary beneficiary. Certain prior year amounts for Tetra Tech, Inc. and its reportable segments have been revised to conform to the current year presentation. In the first quarter of fiscal 2013, we implemented a reorganization of our operations to improve future growth and profitability, including the consolidation and realignment of certain operating activities to achieve efficiencies in our segment management. This reorganization included the elimination of the EAS reportable segment, and the re-assignment of its operations to the ECS and TSS segments (see Note 18, "Reportable Segments" for further discussion). All significant intercompany balances and transactions have been eliminated in consolidation.
Fiscal Year	'
	Fiscal Year.    We report results of operations based on 52 or 53-week periods ending on the Sunday nearest September 30. Fiscal years 2013, 2012 and 2011 each contained 52 weeks.
Use of Estimates	'
	Use of Estimates.    The preparation of financial statements in conformity with U.S. GAAP requires us to make estimates and assumptions. These estimates and assumptions affect the amounts reported in our consolidated financial statements and accompanying notes. Although such estimates and assumptions are based on management's best knowledge of current events and actions we may take in the future, actual results could differ materially from those estimates.
Revenue Recognition and Contract Costs	'
	Revenue Recognition and Contract Costs.    We recognize revenue for most of our contracts using the percentage-of-completion method, primarily based on contract costs incurred to date compared to total estimated contract costs. We generally utilize the cost-to-cost approach to estimate the progress towards completion in order to determine the amount of revenue and profit to recognize. Revenue and cost estimates for each significant contract are reviewed and reassessed quarterly. Changes in those estimates could result in recognition of cumulative catch-up adjustments to the contract's inception-to-date revenue, costs and profit in the period in which such changes are made. Changes in revenue and cost estimates could also result in a projected loss that would be recorded immediately in earnings. For fiscal years 2013, 2012 and 2011, we recognized net favorable (unfavorable) operating income adjustments of ($40.1) million, $0.5 million and ($8.6) million, respectively, due to changes in estimates.
	Certain of our contracts are service-related contracts, such as providing operations and maintenance services or a variety of technical assistance services. Our service contracts are accounted for using the proportional performance method under which revenue is recognized in proportion to the number of service activities performed, in proportion to the direct costs of performing the service activities, or evenly across the period of performance depending upon the nature of the services provided.
	We recognize revenue for work performed under three major types of contracts: fixed-price, time-and-materials and cost-plus.
	Fixed-Price.    We enter into two major types of fixed-price contracts: FFP and FPPU. Under FFP contracts, our clients pay us an agreed fixed-amount negotiated in advance for a specified scope of work. We generally recognize revenue on FFP contracts using the percentage-of-completion method. If the nature or circumstances of the contract prevent us from preparing a reliable estimate at completion, we will delay profit recognition until adequate information about the contract's progress becomes available. Under our FPPU contracts, clients pay us a set fee for each service or production transaction that we complete. Accordingly, we recognize revenue under FPPU contracts as we complete the related service or production transactions, generally using the proportional performance method.
	Time-and-Materials.    Under time-and-materials contracts, we negotiate hourly billing rates and charge our clients based on the actual time that we spend on a project. In addition, clients reimburse us for our actual out-of-pocket costs for materials and other direct incidental expenditures that we incur in connection with our performance under the contract. The majority of our time-and-material contracts are subject to maximum contract values and, accordingly, revenue under these contracts is generally recognized under the percentage-of-completion method. However, time and materials contracts that are service-related contracts are accounted for utilizing the proportional performance method. Revenue on contracts that are not subject to maximum contract values is recognized based on the actual number of hours we spend on the projects plus any actual out-of-pocket costs of materials and other direct incidental expenditures that we incur on the projects. Our time-and-materials contracts also generally include annual billing rate adjustment provisions.
	Cost-Plus.    Under cost-plus contracts, we are reimbursed for allowable or otherwise defined costs incurred plus a fee or mark-up. The contracts may also include incentives for various performance criteria, including quality, timeliness, ingenuity, safety and cost-effectiveness. In addition, our costs are generally subject to review by our clients and regulatory audit agencies, and such reviews could result in costs being disputed as non-reimbursable under the terms of the contract. Revenue for cost-plus contracts is recognized at the time services are performed based upon the amounts we expect to realize using the percentage-of-completion method. Revenue is not recognized for non-recoverable costs. Performance incentives are included in our estimates of revenue when their realization is reasonably assured.
	If estimated total costs on any contract indicate a loss, we recognize the entire estimated loss in the period the loss becomes known. The cumulative effect of revisions to revenue; estimated costs to complete contracts, including penalties, incentive awards, change orders, claims, liquidated damages, anticipated losses, and other revisions are recorded in the period in which the revisions are identified and the loss can be reasonably estimated. Such revisions could occur in any reporting period and the effects may be material depending on the size of the project or the adjustment.
	Once contract performance is underway, we may experience changes in conditions, client requirements, specifications, designs, materials and expectations regarding the period of performance. Such changes are "change orders" and may be initiated by us or by our clients. In many cases, agreement with the client as to the terms of change orders is reached prior to work commencing; however, sometimes circumstances require that work progress without obtaining client agreement. Revenue related to change orders is recognized as costs are incurred. Change orders that are unapproved as to both price and scope are evaluated as claims.
	Claims are amounts in excess of agreed contract prices that we seek to collect from our clients or other third parties for delays, errors in specifications and designs, contract terminations, change orders in dispute or unapproved as to both scope and price, or other causes of unanticipated additional costs. Revenue on claims is recognized only to the extent that contract costs related to the claims have been incurred and when it is probable that the claim will result in a bona fide addition to contract value that can be reliably estimated. No profit is recognized on a claim until final settlement occurs. This can lead to a situation in which costs are recognized in one period and revenue is recognized in a subsequent period when a client agreement is obtained or a claims resolution occurs.
Cash and Cash Equivalents	'
	Cash and Cash Equivalents.    Cash and cash equivalents include all highly liquid investments with maturities of 90 days or less at the date of purchase. Restricted cash of $4.5 million and $5.0 million was included in "Prepaid expenses and other current assets" on consolidated balance sheets at fiscal 2013 and 2012 year-ends, respectively. For cash held by our consolidated joint ventures, see Note 16, "Joint Ventures."
Insurance Matters, Litigation and Contingencies	'
	Insurance Matters, Litigation and Contingencies.    In the normal course of business, we are subject to certain contractual guarantees and litigation. In addition, we maintain insurance coverage for various aspects of our business and operations. We record in our consolidated balance sheets amounts representing our estimated liability for these legal and insurance obligations. We include any adjustments to these liabilities in our consolidated results of operations.
Accounts Receivable - Net	'
	Accounts Receivable – Net.    Net accounts receivable is primarily comprised of billed and unbilled accounts receivable, contract retentions and allowances for doubtful accounts. Billed accounts receivable represent amounts billed to clients that have not been collected. Unbilled accounts receivable represent revenue recognized but not yet billed pursuant to contract terms or billed after the period end date. Most of our unbilled receivables at September 29, 2013 are expected to be billed and collected within 12 months. Unbilled accounts receivable also include amounts related to requests for equitable adjustment to contracts that provide for price redetermination primarily with the U.S. federal government. These amounts are recorded only when they can be reliably estimated and realization is probable. Contract retentions represent amounts withheld by clients until certain conditions are met or the project is completed, which may be several months or years. Allowances for doubtful accounts represent the amounts that may become uncollectible or unrealizable in the future. We determine an estimated allowance for uncollectible accounts based on management's judgment regarding our operating performance related to the adequacy of the services performed and delivered, the status of change orders and claims, our experience settling change orders and claims, and the financial condition of our clients. Billings in excess of costs on uncompleted contracts represent the amounts of cash collected from clients and billings to clients on contracts in advance of work performed and revenue recognized. The majority of these amounts will be earned within 12 months.
Property and Equipment	'
	Property and Equipment.    Property and equipment are recorded at cost and are depreciated over their estimated useful lives using the straight-line method. When property and equipment are retired or otherwise disposed of, the cost and accumulated depreciation are removed from our consolidated balance sheets and any resulting gain or loss is reflected in our consolidated statements of operations. Expenditures for maintenance and repairs are expensed as incurred. Generally, estimated useful lives range from three to ten years for equipment, furniture and fixtures. Buildings are depreciated over periods not exceeding 40 years. Leasehold improvements are amortized on a straight-line basis over the shorter of their estimated useful lives or the length of the lease.
Long-Lived Assets	'
	Long-Lived Assets.    Our policy regarding long-lived assets is to evaluate the recoverability of our assets when the facts and circumstances suggest that the assets may be impaired. This assessment is performed based on the estimated undiscounted cash flows compared to the carrying value of the assets. If the future cash flows (undiscounted and without interest charges) are less than the carrying value, a write-down would be recorded to reduce the related asset to its estimated fair value.
	We recognize a liability for contract termination costs associated with an exit activity for costs that will continue to be incurred under a lease for its remaining term without economic benefit to us, initially measured at its fair value at the cease-use date. The fair value is determined based on the remaining lease rentals, adjusted for the effects of any prepaid or deferred items recognized under the lease, and reduced by estimated sublease rentals.
Business Combinations	'
	Business Combinations.    The cost of an acquired company is assigned to the tangible and intangible assets purchased and the liabilities assumed on the basis of their fair values at the date of acquisition. The determination of fair values of assets and liabilities acquired requires us to make estimates and use valuation techniques when a market value is not readily available. Any excess of purchase price over the fair value of net tangible and intangible assets acquired is allocated to goodwill. Goodwill typically represents the value paid for the assembled workforce and enhancement of our service offerings. Transaction costs associated with business combinations are expensed as they are incurred.
Goodwill and Intangible Assets	'
	Goodwill and Intangible Assets.    Goodwill represents the excess of the aggregate purchase price over the fair value of the net assets acquired in a business acquisition. Following an acquisition, we perform an analysis to value the acquired company's tangible and identifiable intangible assets and liabilities. With respect to identifiable intangible assets, we consider backlog, non-compete agreements, client relations, trade names, patents and other assets. We amortize our intangible assets based on the period over which the contractual or economic benefits of the intangible assets are expected to be realized. We assess the recoverability of the unamortized balance of our intangible assets when indicators of impairment are present based on expected future profitability and undiscounted expected cash flows and their contribution to our overall operations. Should the review indicate that the carrying value is not fully recoverable, the excess of the carrying value over the fair value of the intangible assets would be recognized as an impairment loss.
	We test our goodwill for impairment on an annual basis, and more frequently when an event occurs or circumstances indicate that the carrying value of the asset may not be recoverable. We believe the methodology that we use to review impairment of goodwill, which includes a significant amount of judgment and estimates, provides us with a reasonable basis to determine whether impairment has occurred. However, many of the factors employed in determining whether our goodwill is impaired are outside of our control and it is reasonably likely that assumptions and estimates will change in future periods. These changes could result in future impairments.
	We perform our annual goodwill impairment review at the beginning of our fiscal fourth quarter. Our last annual review at July 1, 2013 (i.e., the first day of our fiscal fourth quarter), indicated that we had no impairment of goodwill, and all of our reporting units had estimated fair values that were in excess of their carrying values, including goodwill. In addition, we regularly evaluate whether events and circumstances have occurred that may indicate a potential change in recoverability of goodwill. We perform interim goodwill impairment reviews between our annual reviews if certain events and circumstances have occurred, including a deterioration in general economic conditions, an increased competitive environment, a change in management, key personnel, strategy or customers, negative or declining cash flows, or a decline in actual or planned revenue or earnings compared with actual and projected results of relevant prior periods (See Note 6, "Goodwill and Intangible Assets" for further discussion). We assess goodwill for impairment at the reporting unit level, which is defined as an operating segment or one level below an operating segment, referred to as a component. Our operating segments are the same as our reportable segments and our reporting units for goodwill impairment testing are the components one level below our reportable segments. These components constitute a business for which discrete financial information is available and where segment management regularly reviews the operating results of that component. We aggregate components within an operating segment that have similar economic characteristics.
	The impairment test for goodwill is a two-step process involving the comparison of the estimated fair value of each reporting unit to the reporting unit's carrying value, including goodwill. We estimate the fair value of reporting units based on a comparison and weighting of the income approach, specifically the discounted cash flow method and the market approach, which estimates the fair value our reporting units based upon comparable market prices and recent transactions and also validates the reasonableness of the multiples from the income approach. If the fair value of a reporting unit exceeds its carrying amount, the goodwill of the reporting unit is not considered impaired; therefore, the second step of the impairment test is unnecessary. If the carrying amount of a reporting unit exceeds its fair value, we perform the second step of the goodwill impairment test to measure the amount of impairment loss to be recorded. If our goodwill is impaired, we are required to record a non-cash charge that could have a material adverse effect on our consolidated financial statements.
Contingent Consideration	'
	Contingent Consideration.    Most of our acquisition agreements include contingent earn-out arrangements, which are generally based on the achievement of future operating income thresholds. The contingent earn-out arrangements are based upon our valuations of the acquired companies and reduce the risk of overpaying for acquisitions if the projected financial results are not achieved. For acquisitions completed prior to fiscal 2010, contingent earn-out payments are accrued as "Estimated earn-out liabilities" when the related operating income thresholds have been achieved, and a corresponding increase in goodwill is recorded. These contingent earn-out payments are reflected as cash flows used in investing activities on the consolidated statements of cash flows in the period paid. At September 29, 2013, there was no contingent consideration remaining for acquisitions completed prior to fiscal 2010 that would be recorded as an addition to goodwill, if earned.
	For acquisitions consummated during or subsequent to fiscal 2010, the fair values of these earn-out arrangements are included as part of the purchase price of the acquired companies on their respective acquisition dates. For each transaction, we estimate the fair value of contingent earn-out payments as part of the initial purchase price and record the estimated fair value of contingent consideration as a liability in "Estimated contingent earn-out liabilities" and "Long-term estimated contingent earn-out liabilities" on the consolidated balance sheets. We consider several factors when determining that contingent earn-out liabilities are part of the purchase price, including the following: (1) the valuation of our acquisitions is not supported solely by the initial consideration paid, and the contingent earn-out formula is a critical and material component of the valuation approach to determining the purchase price; and (2) the former owners of acquired companies that remain as key employees receive compensation other than contingent earn-out payments at a reasonable level compared with the compensation of our other key employees. The contingent earn-out payments are not affected by employment termination.
	We measure our contingent earn-out liabilities at fair value on a recurring basis using significant unobservable inputs classified within Level 3 of the fair value hierarchy. We use a probability weighted discounted income approach as a valuation technique to convert future estimated cash flows to a single present value amount. The significant unobservable inputs used in the fair value measurements are operating income projections over the earn-out period (generally two or three years), and the probability outcome percentages we assign to each scenario. Significant increases or decreases to either of these inputs in isolation would result in a significantly higher or lower liability with a higher liability capped by the contractual maximum of the contingent earn-out obligation. Ultimately, the liability will be equivalent to the amount paid, and the difference between the fair value estimate and amount paid will be recorded in earnings. The amount paid that is less than or equal to the liability on the acquisition date is reflected as cash used in financing activities in our consolidated statements of cash flows. Any amount paid in excess of the liability on the acquisition date is reflected as cash used in operating activities.
	We review and re-assess the estimated fair value of contingent consideration on a quarterly basis, and the updated fair value could differ materially from the initial estimates. Changes in the estimated fair value of our contingent earn-out liabilities related to the time component of the present value calculation are reported in interest expense. Adjustments to the estimated fair value related to changes in all other unobservable inputs are reported in operating income.
Assets Held for Sale	'
	Assets Held for Sale.    Assets that meet the held for sale classification criteria are valued at the lower of their carrying amount or estimated fair value less cost to sell. If the carrying amount of the asset exceeds its estimated fair value less cost to sell, an impairment loss is recognized. Depreciation, depletion and amortization expense is not recorded on assets once they are classified as held for sale.
Fair Value of Financial Instruments	'
	Fair Value of Financial Instruments.    We determine the fair values of our financial instruments, including short-term investments, debt instruments and derivative instruments based on inputs or assumptions that market participants would use in pricing an asset or a liability. We categorize our instruments using a valuation hierarchy for disclosure of the inputs used to measure fair value. This hierarchy prioritizes the inputs into three broad levels as follows: Level 1 inputs are quoted prices (unadjusted) in active markets for identical assets or liabilities; Level 2 inputs are quoted prices for similar assets and liabilities in active markets or inputs that are observable for the asset or liability, either directly or indirectly through market corroboration, for substantially the full term of the financial instrument; and Level 3 inputs are unobservable inputs based on our own assumptions used to measure assets and liabilities at fair value. The classification of a financial asset or liability within the hierarchy is determined based on the lowest level input that is significant to the fair value measurement.
	The carrying amounts of cash and cash equivalents, accounts receivable and accounts payable approximate fair values based on their short-term nature. The carrying amounts of our revolving credit facility approximates fair value because the interest rates are based upon variable reference rates (see Note 9, "Long-Term Debt" and Note 14, "Derivative Financial Instruments" for additional disclosure). Certain other assets and liabilities, such as contingent earn-out liabilities, assets held for sale and amounts related to cash-flow hedges, are required to be carried in our consolidated financial statements at fair value.
	Our fair value measurement methods may produce a fair value calculation that may not be indicative of net realizable value or reflective of future fair values. Although we believe our valuation methods are appropriate and consistent with those used by other market participants, the use of different methodologies or assumptions to determine fair value could result in a different fair value measurement at the reporting date.
Derivative Financial Instruments	'
	Derivative Financial Instruments.    We account for our derivative instruments as either assets or liabilities and carry them at fair value. For derivative instruments that hedge the exposure to variability in expected future cash flows that are designated as cash flow hedges, the effective portion of the gain or loss on the derivative instrument is reported as a component of accumulated other comprehensive income in stockholders' equity and reclassified into income in the same period or periods during which the hedged transaction affects earnings. The ineffective portion of the gain or loss on the derivative instrument, if any, is recognized in current income. To receive hedge accounting treatment, cash flow hedges must be highly effective in offsetting changes to expected future cash flows on hedged transactions.
	The net gain or loss on the effective portion of a derivative instrument that is designated as an economic hedge of the foreign currency translation exposure generated by the re-measurement of certain assets and liabilities denominated in a non-functional currency in a foreign operation is reported in the same manner as a foreign currency translation adjustment. Accordingly, any gains or losses related to these derivative instruments are recognized in current income. Derivatives that do not qualify as hedges are adjusted to fair value through current income.
Deferred Compensation	'
	Deferred Compensation.    We maintain a non-qualified defined contribution supplemental retirement plan for certain key employees that is accounted for in accordance with applicable authoritative guidance on accounting for deferred compensation arrangements where amounts earned are held in a rabbi trust and invested. Employee deferrals and our match are deposited into a rabbi trust, and the funds are generally invested in individual variable life insurance contracts that we own and are specifically designed to informally fund savings plans of this nature. Our consolidated balance sheets reflect our investment in variable life insurance contracts in "Other long-term assets." Our obligation to participating employees is reflected in "Other long-term liabilities." All income and expenses related to the rabbi trust are reflected in our consolidated statements of operations.
Income Taxes	'
	Income Taxes.    We file a consolidated U.S. federal income tax return and a combined California franchise tax return. In addition, we file other returns that are required in the states, foreign jurisdictions and other jurisdictions in which we do business. We account for certain income and expense items differently for financial reporting and income tax purposes. Deferred tax assets and liabilities are computed for the difference between the financial statement and tax bases of assets and liabilities that will result in taxable or deductible amounts in the future based on enacted tax laws and rates applicable to the periods in which the differences are expected to reverse. In determining the need for a valuation allowance, management reviews both positive and negative evidence, including current and historical results of operations, future income projections and potential tax planning strategies. Based on our assessment, we have concluded that a portion of the deferred tax assets at September 29, 2013 will not be realized.
	According to the authoritative guidance on accounting for uncertainty in income taxes, we may recognize the tax benefit from an uncertain tax position only if it is more likely than not that the tax position will be sustained on examination by the taxing authorities based on the technical merits of the position. The tax benefits recognized in the financial statements from such a position should be measured based on the largest benefit that has a greater than 50% likelihood of being realized upon ultimate settlement. This guidance also addresses de-recognition, classification, interest and penalties on income taxes, accounting in interim periods and disclosure requirements for uncertain tax positions.
	In the fourth quarter of fiscal 2013, we identified and corrected errors relating to prior years that increased both income tax expense and our net loss for the year by $3.3 million. The out-of-period adjustments related to book versus tax valuations of fixed assets and acquired intangible assets. Had all adjustments been recorded in their appropriate periods, net loss attributable to Tetra Tech for fiscal 2013 would have increased to net income of $1.1 million. We do not believe that the accounting errors are material to our 2013 annual results or to our previously issued annual or interim financial statements. As a result, we did not adjust any prior period amounts.
Concentration of Credit Risk	'
	Concentration of Credit Risk.    Financial instruments that subject us to credit risk consist primarily of cash and cash equivalents and net accounts receivable. In the event that we have surplus cash, we place our temporary cash investments with lower risk financial institutions and, by policy, limit the amount of investment exposure to any one financial institution. Approximately 22% of accounts receivable were due from various agencies of the U.S. federal government at fiscal 2013 year-end. The remaining accounts receivable are generally diversified due to the large number of organizations comprising our client base and their geographic dispersion. We perform ongoing credit evaluations of our clients and maintain an allowance for potential credit losses. Approximately 47%, 26% and 27% of our fiscal 2013 revenue was generated from our U.S government, U.S. commercial and international clients, respectively (see Note 18, "Reportable Segments" for more information).
Foreign Currency Translation	'
	Foreign Currency Translation.    We determine the functional currency of our foreign operating units based upon the primary currency in which they operate. These operating units maintain their accounting records in their local currency, primarily CAD. Where the functional currency is not the U.S. dollar, translation of assets and liabilities to U.S. dollars is based on exchange rates at the balance sheet date. Translation of revenue and expenses to U.S. dollars is based on the average rate during the period. Translation gains or losses are reported as a component of other comprehensive income (loss). Gains or losses from foreign currency transactions are included in results of operations, with the exception of intercompany foreign transactions that are considered long-term investments, which are recorded in "Accumulated other comprehensive income" on the consolidated balance sheets.

Accounts_Receivable_Net_Tables

Accounts Receivable - Net (Tables)	12 Months Ended
	Sep. 29, 2013
Accounts Receivable - Net	'
Net accounts receivable and billings in excess of costs on uncompleted contracts	'
		Fiscal Year Ended
		September 29,			September 30,
		2013			2012
		(in thousands)
	Billed	$	375,149		$	362,331
	Unbilled		306,969			355,793
	Contract retentions		23,353			17,908
	Total accounts receivable – gross		705,471			736,032
	Allowance for doubtful accounts		(44,624	)		(35,552	)
	Total accounts receivable – net	$	660,847		$	700,480
	Current billings in excess of costs on uncompleted contracts	$	79,507		$	90,909
	Non-current billings in excess of costs on uncompleted contracts		–			4,410
	Total billings in excess of costs on uncompleted contracts	$	79,507		$	95,319

Mergers_and_Acquisitions_Table

Mergers and Acquisitions (Tables)	12 Months Ended
	Sep. 29, 2013
Mergers and Acquisitions	'
Summary of estimated fair values of assets acquired and liabilities assumed	'
		Amount
		(in thousands)
	Current assets	$	77,698
	Property and equipment		7,178
	Goodwill		128,140
	Intangible and other assets		36,988
	Current liabilities		(42,481	)
	Long-term deferred taxes		(9,622	)
	Noncontrolling interests		(12,222	)
	Net assets acquired	$	185,679
Summary of changes in the carrying value of estimated contingent earn-out liabilities	'
		Fiscal Year Ended
		September 29,			September 30,			October 2,
		2013			2012			2011
		(in thousands)
	Beginning balance (at fair value)	$	51,539		$	75,159		$	20,504
	Estimated earn-out liabilities for acquisitions during the fiscal year		75,253			18,981			55,622
	Earn-out liabilities for acquisitions completed prior to fiscal 2010.		250			9,974			21,978
	Increases due to re-measurement of fair value reported in interest expense		2,433			1,374			1,612
	Net decreases due to re-measurement of fair value reported as gains in operating income		(9,560	)		(19,246	)		(1,755	)
	Foreign exchange impact		(2,480	)		3,027			(743	)
	Earn-out payments:
	Reported as cash used in operating activities		(695	)		(601	)		–
	Reported as cash used in investing activities		(1,279	)		(11,773	)		(22,059	)
	Reported as cash used in financing activities		(33,672	)		(18,055	)		–
	Settlement of receivables due from sellers		–			(7,301	)		–
	Ending balance (at fair value)	$	81,789		$	51,539		$	75,159

Goodwill_and_Intangible_Assets1

Goodwill and Intangible Assets (Tables)	12 Months Ended
	Sep. 29, 2013
Goodwill and Intangible Assets	'
Schedule of carrying amount of reporting units including goodwill	'
		ECS			TSS			EAS			RCM			Total
		(in thousands)
	Balance at October 2, 2011	$	405,678		$	82,091		$	17,710		$	63,935		$	569,414
	Inter-segment transfer (1):
	Fiscal 2012 realignment		(29,338	)		45,435			–			(16,097	)		–
	Fiscal 2013 realignment		2,605			15,105			(17,710	)		–			–
	Goodwill additions		15,367			31,236			–			1,945			48,548
	Foreign exchange impact		18,910			–			–			–			18,910
	Goodwill impairment		(914	)		–			–			–			(914	)
	Balance at September 30, 2012		412,308			173,867			–			49,783			635,958
	Goodwill additions		14,364			3,594			–			145,064			163,022
	Foreign exchange impact		(16,464	)		118			–			(3,242	)		(19,588	)
	Goodwill impairment		(56,600	)		–			–			–			(56,600	)
	Balance at September 29, 2013	$	353,608		$	177,579		$	–		$	191,605		$	722,792
	-1
	Prior-year amounts have been reclassified due to realignment of certain operating activities in our reportable segments in fiscal 2012 and fiscal 2013. See Note 18, "Reportable Segments" for further information.
Summary of changes in the carrying value of goodwill	'
		June 30, 2013
		TTC			GMP			AMT
		(in thousands)
	Carrying value before impairment	$	245,634		$	116,184		$	56,474
	Goodwill impairment		(27,900	)		(11,900	)		(16,800	)
	Carrying value after impairment	$	217,734		$	104,284		$	39,674
Summary of acquired identifiable intangible assets with finite useful lives	'
		Fiscal Year Ended
		September 29, 2013									September 30, 2012
		Weighted-			Gross			Accumulated			Gross			Accumulated
		Average			Amount			Amortization			Amount			Amortization
		Remaining
		Life
		(in years)
		($ in thousands)
	Non-compete agreements		2.6		$	6,160		$	(5,247	)	$	5,467		$	(4,685	)
	Client relations		4.5			128,839			(49,189	)		99,096			(31,477	)
	Backlog		0.4			68,968			(64,675	)		59,931			(55,908	)
	Technology and trade names		2.9			4,204			(2,131	)		3,034			(1,227	)
	Total				$	208,171		$	(121,242	)	$	167,528		$	(93,297	)
Estimated amortization expense for the succeeding five years and beyond	'
		Amount
		(in thousands)
	2014	$	26,815
	2015		19,328
	2016		15,875
	2017		13,626
	2018		6,333
	Beyond		4,952
	Total	$	86,929

Property_and_Equipment_Tables

Property and Equipment (Tables)	12 Months Ended
	Sep. 29, 2013
Property and Equipment	'
Schedule of components of property and equipment	'
		Fiscal Year Ended
		September 29,			September 30,
		2013			2012
		(in thousands)
	Land and buildings	$	5,565		$	5,537
	Equipment, furniture and fixtures		210,172			177,710
	Leasehold improvements		26,429			26,180
	Total property and equipment		242,166			209,427
	Accumulated depreciation		(154,140	)		(135,118	)
	Property and equipment, net	$	88,026		$	74,309

Income_Taxes_Tables

Income Taxes (Tables)	12 Months Ended
	Sep. 29, 2013
Income Taxes	'
Schedule of income before income taxes, by geographical area	'
		Fiscal Year Ended
		September 29,			September 30,			October 2,
		2013			2012			2011
		(in thousands)
	Income (loss) before income taxes:
	United States	$	60,547		$	141,035		$	126,912
	Foreign		(48,015	)		19,761			13,580
	Total income before income taxes	$	12,532		$	160,796		$	140,492
Schedule of components of income tax expense	'
		Fiscal Year Ended
		September 29,			September 30,			October 2,
		2013			2012			2011
		(in thousands)
	Current:
	Federal	$	11,155		$	46,058		$	30,246
	State		2,705			6,949			5,948
	Foreign		11,646			8,569			9,596
	Total current income tax expense		25,506			61,576			45,790
	Deferred:
	Federal		(2,965	)		(200	)		6,755
	State		(637	)		(622	)		1,069
	Foreign		(7,866	)		(4,690	)		(6,104	)
	Total deferred income tax expense (benefit)		(11,468	)		(5,512	)		1,720
	Total income tax expense	$	14,038		$	56,064		$	47,510
Schedule of reconciliation of income tax expense and effective income tax rates	'
		Fiscal Year Ended
		September 29, 2013						September 30, 2012					October 2, 2011
		($ in thousands)
	Tax at federal statutory rate	$	4,386			35	%	$	56,278		35	%	$	49,172		35	%
	State taxes, net of federal benefit		1,316			10.5			4,932		3.1			4,376		3.1
	R&E credits		(6,622	)		(52.8	)		(360	)	(0.2	)		(1,689	)	(1.2	)
	Domestic production deduction		(828	)		(6.6	)		(774	)	(0.5	)		(770	)	(0.6	)
	Tax differential on foreign earnings		(4,263	)		(34.0	)		(4,444	)	(2.8	)		(4,140	)	(3.0	)
	Corrections of prior-year errors		3,255			26			–		–			–		-
	Goodwill and contingent consideration		11,288			90			(1,552	)	(1.0	)		–		-
	Stock compensation		443			3.5			80		0.1			301		0.2
	Valuation allowance		4,947			39.5			2,512		1.6			-		-
	Other		116			0.9			(608	)	(0.4	)		260		0.3
	Total income tax expense	$	14,038			112	%	$	56,064		34.9	%	$	47,510		33.8	%
Schedule of temporary differences comprising the net deferred income tax liability	'
		Fiscal Year Ended
		September 29,			September 30,
		2013			2012
		(in thousands)
	Deferred Tax Asset:
	State taxes	$	452		$	975
	Reserves and contingent liabilities		5,883			4,689
	Allowance for doubtful accounts		7,345			2,039
	Accrued liabilities		14,425			13,298
	Stock-based compensation		10,778			10,980
	Loss carry-forwards		9,563			2,926
	Valuation allowance on loss carry-forwards		(7,459	)		(2,512	)
	Total deferred tax asset		40,987			32,395
	Deferred Tax Liability:
	Unbilled revenue		(47,281	)		(45,417	)
	Prepaid expense		(7,522	)		(2,251	)
	Intangibles		(24,933	)		(21,695	)
	Property and equipment		(9,946	)		(8,109	)
	Total deferred tax liability		(89,682	)		(77,472	)
	Net deferred tax liability	$	(48,695	)	$	(45,077	)
Reconciliation of the beginning and ending amounts of unrecognized tax benefits	'
		Fiscal Year Ended
		September 29,			September 30,			October 2,
		2013			2012			2011
		(in thousands)
	Beginning balance	$	24,092		$	25,940		$	21,806
	Additions for current year tax positions		2,661			6,273			8,007
	Additions for prior year tax positions		4,951			19			2,554
	Reductions for prior year tax positions		(5,818	)		(8,072	)		(6,315	)
	Settlements		–			(68	)		(112	)
	Ending balance	$	25,886		$	24,092		$	25,940

LongTerm_Debt_Tables

Long-Term Debt (Tables)	12 Months Ended
	Sep. 29, 2013
Long-Term Debt	'
Schedule of long-term debt	'
		Fiscal Year Ended
		September 29,			September 30,
		2013			2012
		(in thousands)
	Credit facilities	$	205,000		$	79,233
	Other		2,749			3,845
	Total long-term debt		207,749			83,078
	Less: Current portion of long-term debt		(4,311	)		(2,031	)
	Long-term debt, less current portion	$	203,438		$	81,047
Schedule of maturities of long-term debt	'
		Amount
		(in thousands)
	2014	$	4,311
	2015		10,868
	2016		15,753
	2017		15,379
	2018		161,438
	Total	$	207,749

Leases_Tables

Leases (Tables)	12 Months Ended
	Sep. 29, 2013
Leases	'
Schedule of amounts payable under non-cancelable operating and capital lease commitments	'
		Operating			Capital
		(in thousands)
	2014	$	71,293		$	874
	2015		54,169			648
	2016		35,265			384
	2017		23,898			5
	2018		14,771			–
	Beyond		24,628			–
	Total	$	224,024			1,911
	Less: Amounts representing interest					(132	)
	Net present value				$	1,779
Schedule of reconciliation of the beginning and ending balances of liabilities related to lease contract termination costs	'
		ECS			TSS			Total
		(in thousands)
	Balance at September 30, 2012	$	–		$	2,940		$	2,940
	Costs incurred and charged to expense		3,744			1,055			4,799
	Adjustments (1)		(34	)		(1,432	)		(1,466	)
	Balance at September 29, 2013	$	3,710		$	2,563		$	6,273
	-1
	Adjustments of the actual timing and potential termination costs or realization of sublease income.

Stockholders_Equity_and_Stock_1

Stockholders' Equity and Stock Compensation Plans (Tables)	12 Months Ended
	Sep. 29, 2013
Stockholders' Equity and Stock Compensation Plans	'
Schedule of the stock-based compensation and related income tax benefits	'
		Fiscal Year Ended
		September 29,			September 30,			October 2,
		2013			2012			2011
		(in thousands)
	Total stock-based compensation	$	8,775		$	10,839		$	10,582
	Income tax benefit related to stock-based compensation		(3,048	)		(4,288	)		(3,804	)
	Stock-based compensation, net of tax benefit	$	5,727		$	6,551		$	6,778
Schedule of stock option activity	'
		Number of			Weighted-			Weighted-			Aggregate
		Options			Average			Average			Intrinsic Value
		(in thousands)			Exercise Price			Remaining			(in thousands)
					per Share			Contractual Term
								(in years)
	Outstanding on September 30, 2012		4,876		$	21.5
	Granted		287			24.24
	Exercised		(793	)		20.18
	Cancelled		(106	)		23.72
	Outstanding at September 29, 2013		4,264		$	21.88			3.7		$	17,502
	Vested or expected to vest at September 29, 2013		4,229		$	21.9			3.7		$	17,268
	Exercisable on September 29, 2013		3,079		$	21.16			5.4		$	14,829
Schedule of assumptions used in the calculation of the fair value of stock options using the Black-Scholes option pricing model	'
		Fiscal Year Ended
		September 29,			September 30,			October 2,
		2013			2012			2011
	Dividend yield		–			–			–
	Expected stock price volatility		41.7% - 42.2%			41.9% - 44.0%			41.8 - 42.7%
	Risk-free rate of return, annual		0.6% - 1.3%			0.7% - 1.1%			1.3 - 2.1%
Schedule of restricted stock activity	'
		Number of			Weighted-
		Shares			Average Grant
		(in thousands)			Date Fair
					Value
	Nonvested balance at September 30, 2012		190		$	23.08
	Granted		113			24.23
	Vested		(96	)		23.45
	Forfeited		(4	)		22.85
	Nonvested balance at September 29, 2013		203		$	23.55
	Vested or expected to vest at September 29, 2013		203		$	23.55
Schedule of RSU activity	'
		Number of			Weighted-
		Shares			Average Grant
		(in thousands)			Date Fair
					Value
	Nonvested balance at September 30, 2012		172		$	22.53
	Granted		227			24.32
	Vested		(43	)		22.53
	Forfeited		(23	)		23.27
	Nonvested balance at September 29, 2013		333		$	23.7
Summary of shares purchased, weighted-average purchase price, cash received, and the aggregate intrinsic value for shares purchased under the ESPP	'
		Fiscal Year Ended
		September 29,			September 30,			October 2,
		2013			2012			2011
		(in thousands, except for purchase price)
	Shares purchased		253			289			246
	Weighted-average purchase price	$	21.96		$	18.35		$	21.3
	Cash received from exercise of purchase rights	$	5,551		$	5,300		$	5,249
	Aggregate intrinsic value	$	1,140		$	935		$	926
Schedule of the assumptions used in the Black-Scholes option pricing model in estimating the grant date fair value of each award granted under the ESPP	'
		Fiscal Year Ended
		September 29,			September 30,			October 2,
		2013			2012			2011
	Dividend yield		-			-			-
	Expected stock price volatility		27.1	%		34.7	%		38	%
	Risk-free rate of return, annual		0.1	%		0.1	%		0.3	%
	Expected life (in years)		1			1			1

Earnings_Per_Share_Tables

Earnings Per Share (Tables)	12 Months Ended
	Sep. 29, 2013
Earnings Per Share	'
Schedule of number of weighted-average shares used to compute basic and diluted EPS	'
		Fiscal Year Ended
		September 29,			September 30,		October 2,
		2013			2012		2011
		(in thousands, except per share data)
	Net income (loss) attributable to Tetra Tech	$	(2,141	)	$	104,380	$	90,039
	Weighted-average common shares outstanding – basic		64,544			63,217		62,053
	Effect of diluted stock options and unvested restricted stock		–			717		722
	Weighted-average common stock outstanding – diluted		64,544			63,934		62,775
	Net income (loss) attributable to Tetra Tech per share:
	Basic	$	(0.03	)	$	1.65	$	1.45
	Diluted	$	(0.03	)	$	1.63	$	1.43

Derivative_Financial_Instrumen1

Derivative Financial Instruments (Tables)	12 Months Ended
	Sep. 29, 2013
Derivative Financial Instruments	'
Schedule of notional principal, fixed rates and related expiration dates of outstanding interest rate swap agreements	'
	Notional Amount				Fixed			Expiration
	(in thousands)				Rate			Date
	$		51,250			1.36	%		May-18
			51,250			1.34	%		May-18
			51,250			1.35	%		May-18
Schedule of fair values of outstanding derivative instruments	'
	The fair values of our outstanding derivative instruments were as follows (in thousands):
				Fair Value of Derivative
				Instruments as of
		Balance Sheet Location		September 29,			September 30,
				2013			2012
	Derivatives designated as hedging instruments:
	Foreign currency forward contracts	Other current liabilities		$	–		$	348
	Interest rate swap agreements	Other current liabilities			987			–
	Total			$	987		$	348

Reportable_Segments_Tables

Reportable Segments (Tables)	12 Months Ended
	Sep. 29, 2013
Reportable Segments	'
Summarized financial information of reportable segments	'
	Reportable Segments
		Fiscal Year Ended
		September 29,			September 30,			October 2,
		2013			2012			2011
		(in thousands)
	Revenue
	ECS	$	1,035,983		$	1,155,256		$	1,110,060
	TSS		932,375			1,020,779			995,249
	RCM		725,689			621,957			604,651
	Elimination of inter-segment revenue		(80,292	)		(86,917	)		(136,816	)
	Total revenue	$	2,613,755		$	2,711,075		$	2,573,144
	Operating Income
	ECS	$	44,598		$	96,220		$	99,868
	TSS		71,842			71,767			69,977
	RCM		(6,706	)		22,374			13,183
	Corporate (1)		(89,516	)		(23,994	)		(36,606	)
	Total operating income	$	20,218		$	166,367		$	146,422
	Depreciation
	ECS	$	10,494		$	10,126		$	12,067
	TSS		2,839			3,227			2,028
	RCM		13,160			10,233			10,101
	Corporate		3,055			3,065			2,941
	Total depreciation	$	29,548		$	26,651		$	27,137
	-1
	Includes goodwill impairment charge, amortization of intangibles, other costs and other income not allocable to segments. The goodwill impairment charge of $56.6 million for fiscal 2013 was recorded at Corporate. The intangible asset amortization expense for fiscal 2013, 2012 and 2011 was $32.4 million, $29.6 million and $28.0 million, respectively. Corporate results also included income for fair value adjustments to contingent consideration liabilities of $9.6 million, $19.2 million and $1.8 million for 2013, 2012 and 2011, respectively.
		September 29,			September 30,
		2013			2012
		(in thousands)
	Total Assets
	ECS	$	912,996		$	915,571
	TSS		673,864			638,405
	RCM		435,053			311,051
	Corporate (1)		(222,821	)		(193,997	)
	Total assets	$	1,799,092		$	1,671,030
	-1
	Corporate assets consist of intercompany eliminations and assets not allocated to segments including goodwill, intangible assets, deferred income taxes and certain other assets.
Schedule of geographic information	'
	Geographic Information
		Fiscal Year Ended
		September 29, 2013						September 30, 2012					October 2, 2011
		Revenue			Long-Lived			Revenue			Long-Lived		Revenue		Long-Lived
					Assets (2)						Assets (2)				Assets (2)
								(in thousands)
	United States	$	1,915,780		$	110,313		$	2,046,700		$	100,958	$	1,976,452	$	102,316
	Foreign countries (1)		697,975			90,435			664,375			70,010		596,692		78,198
	-1
	Includes revenue generated from our foreign operations, primarily in Canada, and revenue generated from non-U.S. clients. Long-lived assets consist primarily of amounts from our Canadian operations.
	-2
	Excludes goodwill and intangible assets.
Summary of revenue by client sector	'
		Fiscal Year Ended
		September 29,			September 30,			October 2,
		2013			2012			2011
		(in thousands)
	Client Sector
	International (1)	$	697,975		$	664,375		$	596,692
	U.S commercial		693,677			718,457			577,782
	U.S. federal government (2)		829,790			1,008,424			1,115,729
	U.S. state and local government		392,313			319,819			282,941
	Total	$	2,613,755		$	2,711,075		$	2,573,144
	-1
	Includes revenue generated from foreign operations, primarily in Canada, and revenue generated from non-U.S. clients.
	-2
	Includes revenue generated under U.S. federal government contracts performed outside the United States.

Quarterly_Financial_Informatio1

Quarterly Financial Information-Unaudited (Tables)	12 Months Ended
	Sep. 29, 2013
Quarterly Financial Information-Unaudited	'
Schedule of unaudited quarterly data	'
		First		Second		Third			Fourth
		Quarter		Quarter		Quarter			Quarter
		(in thousands, except per share data)
	Fiscal Year 2013
	Revenue	$	658,545	$	641,999	$	614,835		$	698,376
	Operating income (loss)		41,809		37,667		(99,884	)		40,626
	Net income (loss) attributable to Tetra Tech		26,224		24,820		(78,385	)		25,200
	Net income (loss) attributable to Tetra Tech per share (1):
	Basic	$	0.41	$	0.38	$	(1.21	)	$	0.39
	Diluted	$	0.41	$	0.38	$	(1.21	)	$	0.39
	Weighted-average common shares outstanding:
	Basic		63,864		64,551		64,832			64,272
	Diluted		64,608		65,472		64,832			64,853
	Fiscal Year 2012
	Revenue	$	682,627	$	624,345	$	684,698		$	719,405
	Operating income		36,093		35,543		46,261			48,470
	Net income attributable to Tetra Tech		22,610		22,284		29,054			30,432
	Earnings per share attributable to Tetra Tech (1):
	Basic	$	0.36	$	0.35	$	0.46		$	0.48
	Diluted	$	0.36	$	0.35	$	0.45		$	0.47
	Weighted-average common shares outstanding:
	Basic		62,433		63,072		63,387			63,623
	Diluted		63,068		63,817		64,179			64,396
	-1
	The sum of the quarterly EPS may not add up to the full-year EPS due to rounding.

Basis_of_Presentation_and_Prep2

Basis of Presentation and Preparation (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Sep. 29, 2013	Sep. 30, 2012	Oct. 02, 2011
	item
Principles of Consolidation and Presentation	'	'	'
Number of weeks in a fiscal year, low end of range	'364 days	'	'
Number of weeks in a fiscal year, high end of range	'371 days	'	'
Number of weeks in a fiscal year	'364 days	'364 days	'364 days
Revenue Recognition and Contract Costs	'	'	'
Number of types of contracts under which revenue is recognized	3	'	'
Number of types of fixed-price contracts	2	'	'
Cash and Cash Equivalents	'	'	'
Maximum term of original maturity to classify instrument as cash equivalent	'90 days	'	'
Restricted cash included in "Prepaid expenses and other current assets"	$4.50	$5	'
Accounts Receivable - Net	'	'	'
Period for billing and collecting unbilled receivables	'12 months	'	'
Period for earning majority of billings in excess of costs	'12 months	'	'
Equipment | Maximum	'	'	'
Estimated useful lives	'	'	'
Estimated useful lives	'10 years	'	'
Equipment | Minimum	'	'	'
Estimated useful lives	'	'	'
Estimated useful lives	'3 years	'	'
Furniture and fixtures | Maximum	'	'	'
Estimated useful lives	'	'	'
Estimated useful lives	'10 years	'	'
Furniture and fixtures | Minimum	'	'	'
Estimated useful lives	'	'	'
Estimated useful lives	'3 years	'	'
Buildings | Maximum	'	'	'
Estimated useful lives	'	'	'
Estimated useful lives	'40 years	'	'

Basis_of_Presentation_and_Prep3

Basis of Presentation and Preparation (Details 2) (USD $)	3 Months Ended	12 Months Ended
	Jun. 30, 2013	Sep. 29, 2013	Sep. 30, 2012
		item
Goodwill and Intangible Assets	'	'	'
Impairment of goodwill	$56,600,000	$56,600,000	$914,000
Number of levels below reportable segments at which goodwill impairment testing is performed	'	1	'
Number of steps involved in process of goodwill annual impairment test	'	2	'
Contingent Consideration	'	'	'
Contingent consideration remaining for acquisitions	'	81,800,000	'
Prior to 2010 acquisitions	'	'	'
Contingent Consideration	'	'	'
Contingent consideration remaining for acquisitions	'	$0	'

Basis_of_Presentation_and_Prep4

Basis of Presentation and Preparation (Details 3)	12 Months Ended
	Sep. 29, 2013
	item
Concentration of Credit Risk	'
Financial institutions, in any such number of which investment exposure is limited	1
Accounts receivable due from various agencies of the U.S. federal government (as a percent)	22.00%
International	'
Concentration of Credit Risk	'
Revenue from customers within risk category	27.00%
U.S. commercial	'
Concentration of Credit Risk	'
Revenue from customers within risk category	26.00%
U.S. federal government	'
Concentration of Credit Risk	'
Revenue from customers within risk category	47.00%
Minimum	'
Concentration of Credit Risk	'
Period for contingent earn-out payments	'2 years
Maximum	'
Concentration of Credit Risk	'
Period for contingent earn-out payments	'3 years

Basis_of_Presentation_and_Prep5

Basis of Presentation and Preparation (Details 4) (USD $)	3 Months Ended								12 Months Ended
In Thousands, unless otherwise specified	Sep. 29, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 30, 2012	Sep. 30, 2012	Jul. 01, 2012	Apr. 01, 2012	Jan. 01, 2012	Sep. 29, 2013	Sep. 30, 2012	Oct. 02, 2011
Revenue recognition and contract costs	'	'	'	'	'	'	'	'	'	'	'
Favorable (unfavorable) operating income adjustments	$40,626	($99,884)	$37,667	$41,809	$48,470	$46,261	$35,543	$36,093	$20,218	$166,367	$146,422
Contracts accounted for under the percentage-of-completion method of accounting	'	'	'	'	'	'	'	'	'	'	'
Revenue recognition and contract costs	'	'	'	'	'	'	'	'	'	'	'
Favorable (unfavorable) operating income adjustments	'	'	'	'	'	'	'	'	($40,100)	$500	($8,600)

Basis_of_Presentation_and_Prep6

Basis of Presentation and Preparation (Details 5) (USD $)	3 Months Ended								12 Months Ended
	Sep. 29, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 30, 2012	Sep. 30, 2012	Jul. 01, 2012	Apr. 01, 2012	Jan. 01, 2012	Sep. 29, 2013	Sep. 30, 2012	Oct. 02, 2011
Income taxes	'	'	'	'	'	'	'	'	'	'	'
Increase in the income tax expense for out-of-period adjustment	'	'	'	'	'	'	'	'	$14,038,000	$56,064,000	$47,510,000
Increase in, net loss expense for out-of-period adjustment	-25,200,000	78,385,000	-24,820,000	-26,224,000	-30,432,000	-29,054,000	-22,284,000	-22,610,000	2,141,000	-104,380,000	-90,039,000
Prior year adjustment	'	'	'	'	'	'	'	'	'	'	'
Income taxes	'	'	'	'	'	'	'	'	'	'	'
Increase in the income tax expense for out-of-period adjustment	3,300,000	'	'	'	'	'	'	'	'	'	'
Increase in, net loss expense for out-of-period adjustment	3,300,000	'	'	'	'	'	'	'	'	'	'
Net income, if prior period adjustments had been recorded in the appropriate periods	$1,100,000	'	'	'	'	'	'	'	'	'	'

Stock_Repurchase_Program_Detai

Stock Repurchase Program (Details) (USD $)	1 Months Ended	12 Months Ended
In Millions, except Share data, unless otherwise specified	Jun. 30, 2013	Sep. 29, 2013
Stock Repurchase Program	'	'
Maximum repurchase amount under stock repurchase program	$100	'
Shares repurchased through open market purchases	'	855,200
Average price of shares repurchased (in dollars per share)	'	$23.39
Cost of shares repurchased	'	$20

Accounts_Receivable_Net_Detail

Accounts Receivable - Net (Details) (USD $)	12 Months Ended
	Sep. 29, 2013	Sep. 30, 2012
	item	item
Accounts Receivable - Net	'	'
Billed	$375,149,000	$362,331,000
Unbilled	306,969,000	355,793,000
Contract retentions	23,353,000	17,908,000
Total accounts receivable - gross	705,471,000	736,032,000
Allowance for doubtful accounts	-44,624,000	-35,552,000
Total accounts receivable - net	660,847,000	700,480,000
Current billings in excess of costs on uncompleted contracts	79,507,000	90,909,000
Non-current billings in excess of costs on uncompleted contracts	'	4,410,000
Total billings in excess of costs on uncompleted contracts	79,507,000	95,319,000
Period for billing and collecting unbilled receivables	'12 months	'
Period for earning majority of billings in excess of costs	'12 months	'
Unbilled accounts receivable related to claims and requests for equitable adjustment on contracts	41,000,000	21,000,000
Loss on uncollectible accounts receivable related to claims	30,200,000	0
Billed accounts receivable related to U.S. federal government contracts	50,500,000	65,900,000
U.S. federal government unbilled receivables, net of progress payments	$79,300,000	$100,400,000
Number of clients exceeding threshold	0	0
Threshold percentage for disclosure of accounts receivable from a single client	10.00%	10.00%

Mergers_and_Acquisitions_Detai

Mergers and Acquisitions (Details) (USD $)	12 Months Ended									3 Months Ended
	Sep. 29, 2013	Sep. 30, 2012	Oct. 02, 2011	Sep. 29, 2013	Sep. 30, 2012	Oct. 02, 2011	Sep. 29, 2013	Sep. 30, 2012	Oct. 02, 2011	Jan. 02, 2011	Oct. 02, 2011
				Prior to 2010 acquisitions	Prior to 2010 acquisitions	Prior to 2010 acquisitions	AEG, Parkland and other 2013 acquisitions	2012 acquisitions	2011 acquisitions	BPR, Inc.	BPR, Inc.
Business acquisition	'	'	'	'	'	'	'	'	'	'	'
Estimated fair value of purchase prices	'	'	'	'	'	'	$248,900,000	$63,200,000	$100,000,000	'	$185,679,000
Initial payment	'	'	'	'	'	'	171,600,000	42,200,000	68,700,000	'	157,000,000
Estimated fair value of contingent earn-out obligations	81,800,000	'	'	0	'	'	75,300,000	19,000,000	26,900,000	'	28,700,000
Estimated contingent earn-out payments of aggregate maximum upon achievement of specified financial objectives	101,300,000	'	'	'	'	'	86,700,000	20,000,000	32,300,000	'	39,200,000
Period for contingent earn-out payments	'	'	'	'	'	'	'	'	'	'2 years	'
Estimated fair values of the assets acquired and liabilities assumed	'	'	'	'	'	'	'	'	'	'	'
Current assets	'	'	'	'	'	'	'	'	'	'	77,698,000
Property and equipment	'	'	'	'	'	'	'	'	'	'	7,178,000
Goodwill	'	'	'	'	'	'	'	'	'	'	128,140,000
Intangible and other assets	'	'	'	'	'	'	'	'	'	'	36,988,000
Current liabilities	'	'	'	'	'	'	'	'	'	'	-42,481,000
Long-term deferred taxes	'	'	'	'	'	'	'	'	'	'	-9,622,000
Noncontrolling interests	'	'	'	'	'	'	'	'	'	'	-12,222,000
Net assets acquired	'	'	'	'	'	'	248,900,000	63,200,000	100,000,000	'	185,679,000
Amount accrued	'	'	'	'	'	'	'	2,000,000	4,500,000	'	'
Liabilities	'	'	'	'	'	'	2,000,000	'	'	'	'
Estimated contingent earn-out liabilities	'	'	'	'	'	'	'	'	'	'	'
Beginning balance (at fair value)	51,539,000	75,159,000	20,504,000	'	'	'	'	'	'	'	'
Estimated earn-out liabilities for acquisition during the fiscal year	75,253,000	18,981,000	55,622,000	250,000	9,974,000	21,978,000	'	'	'	'	'
Increases due to re-measurement of fair value reported in interest expense	2,433,000	1,374,000	1,612,000	'	'	'	'	'	'	'	'
Net decreases due to re-measurement of fair value reported as gains in operating income	-9,560,000	-19,246,000	-1,755,000	'	'	'	'	'	'	'	'
Foreign exchange impact	-2,480,000	3,027,000	-743,000	'	'	'	'	'	'	'	'
Earn-out payments	'	'	'	'	'	'	'	'	'	'	'
Reported as cash used in operating activities	-695,000	-601,000	'	'	'	'	'	'	'	'	'
Reported as cash used in investing activities	-1,279,000	-11,773,000	-22,059,000	'	'	'	'	'	'	'	'
Reported as cash used in financing activities	-33,672,000	-18,055,000	'	'	'	'	'	'	'	'	'
Settlement of receivables due from sellers	'	-7,301,000	'	'	'	'	'	'	'	'	'
Ending balance (at fair value)	$81,789,000	$51,539,000	$75,159,000	'	'	'	'	'	'	'	'

Goodwill_and_Intangible_Assets2

Goodwill and Intangible Assets (Details) (USD $)

3 Months Ended

12 Months Ended

3 Months Ended

12 Months Ended

3 Months Ended

9 Months Ended

12 Months Ended

12 Months Ended

Jun. 30, 2013

Sep. 29, 2013

Sep. 30, 2012

Jun. 30, 2013

Sep. 29, 2013

Sep. 30, 2012

Sep. 30, 2012

Sep. 30, 2012

Jun. 30, 2013

Jun. 30, 2013

Jun. 30, 2013

Jun. 30, 2013

Sep. 29, 2013

Sep. 30, 2012

Sep. 30, 2012

Sep. 30, 2012

Oct. 02, 2011

Sep. 30, 2012

Sep. 29, 2013

Sep. 30, 2012

Sep. 30, 2012

ECS

ECS

ECS

ECS

ECS

TTC, GMP and AMT

TTC

GMP

AMT

TSS

TSS

TSS

TSS

Engineering and architecture services (EAS)

Engineering and architecture services (EAS)

RCM

RCM

RCM

item

Fiscal 2012 realignment

Fiscal 2013 realignment

Fiscal 2012 realignment

Fiscal 2013 realignment

Fiscal 2013 realignment

Fiscal 2012 realignment

Goodwill

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Balance at beginning of the period

'

$635,958,000

$569,414,000

'

$412,308,000

$405,678,000

'

'

'

'

'

'

$173,867,000

$82,091,000

'

'

$17,710,000

'

$49,783,000

$63,935,000

'

Inter-segment transfer, net

'

'

'

'

'

'

-29,338,000

2,605,000

'

'

'

'

'

'

45,435,000

15,105,000

'

-17,710,000

'

'

-16,097,000

Goodwill additions

'

163,022,000

48,548,000

'

14,364,000

15,367,000

'

'

'

'

'

'

3,594,000

31,236,000

'

'

'

'

145,064,000

1,945,000

'

Foreign exchange impact

'

-19,588,000

18,910,000

'

-16,464,000

18,910,000

'

'

'

'

'

'

118,000

'

'

'

'

'

-3,242,000

'

'

Goodwill impairment

-56,600,000

-56,600,000

-914,000

'

-56,600,000

-914,000

'

'

-56,600,000

-27,900,000

-11,900,000

-16,800,000

'

'

'

'

'

'

'

'

'

Balance at end of the period

'

722,792,000

635,958,000

'

353,608,000

412,308,000

'

'

'

111,100,000

72,300,000

32,600,000

177,579,000

173,867,000

'

'

17,710,000

'

191,605,000

49,783,000

'

Gross amounts of goodwill

'

'

'

'

411,100,000

413,200,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Number of reporting units for which goodwill impairment test is triggered

'

'

'

3

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Goodwill impairment, net of tax

'

'

'

'

'

'

'

'

48,100,000

'

'

'

'

'

'

'

'

'

'

'

'

Carrying value before impairment

'

'

'

'

'

'

'

'

'

245,634,000

116,184,000

56,474,000

'

'

'

'

'

'

'

'

'

Carrying value after impairment

'

'

'

'

'

'

'

'

'

$217,734,000

$104,284,000

$39,674,000

'

'

'

'

'

'

'

'

'

Goodwill_and_Intangible_Assets3

Goodwill and Intangible Assets (Details 2) (USD $)	12 Months Ended
	Sep. 29, 2013	Sep. 30, 2012	Oct. 02, 2011
Finite-lived intangible assets	'	'	'
Gross Amount	$208,171,000	$167,528,000	'
Accumulated Amortization	-121,242,000	-93,297,000	'
Amortization expense for intangible assets	32,400,000	29,600,000	28,000,000
Estimated amortization expense	'	'	'
2014	26,815,000	'	'
2015	19,328,000	'	'
2016	15,875,000	'	'
2017	13,626,000	'	'
2018	6,333,000	'	'
Beyond	4,952,000	'	'
Total	86,929,000	'	'
Non-compete agreements	'	'	'
Finite-lived intangible assets	'	'	'
Weighted-Average Remaining Life	'2 years 7 months 6 days	'	'
Gross Amount	6,160,000	5,467,000	'
Accumulated Amortization	-5,247,000	-4,685,000	'
Client relations	'	'	'
Finite-lived intangible assets	'	'	'
Weighted-Average Remaining Life	'4 years 6 months	'	'
Gross Amount	128,839,000	99,096,000	'
Accumulated Amortization	-49,189,000	-31,477,000	'
Backlog	'	'	'
Finite-lived intangible assets	'	'	'
Weighted-Average Remaining Life	'4 months 24 days	'	'
Gross Amount	68,968,000	59,931,000	'
Accumulated Amortization	-64,675,000	-55,908,000	'
Technology and trade names	'	'	'
Finite-lived intangible assets	'	'	'
Weighted-Average Remaining Life	'2 years 10 months 24 days	'	'
Gross Amount	4,204,000	3,034,000	'
Accumulated Amortization	($2,131,000)	($1,227,000)	'

Property_and_Equipment_Details

Property and Equipment (Details) (USD $)	12 Months Ended
	Sep. 29, 2013	Sep. 30, 2012	Oct. 02, 2011
Property and Equipment	'	'	'
Property and equipment at cost, gross	$242,166,000	$209,427,000	'
Accumulated depreciation	-154,140,000	-135,118,000	'
Property and equipment, net	88,026,000	74,309,000	'
Depreciation expense related to property and equipment, including assets under capital leases	29,548,000	26,651,000	27,137,000
Prepaid expenses and other current assets	'	'	'
Property and Equipment	'	'	'
Carrying value of property	'	2,400,000	'
Selling, general and administrative expenses	'	'	'
Property and Equipment	'	'	'
Non-cash impairment charge	'	3,400,000	'
Land and buildings	'	'	'
Property and Equipment	'	'	'
Property and equipment at cost, gross	5,565,000	5,537,000	'
Carrying value of properties held for sale	'	5,800,000	'
Equipment, furniture and fixtures	'	'	'
Property and Equipment	'	'	'
Property and equipment at cost, gross	210,172,000	177,710,000	'
Leasehold improvements	'	'	'
Property and Equipment	'	'	'
Property and equipment at cost, gross	$26,429,000	$26,180,000	'

Property_and_Equipment_Details1

Property and Equipment (Details 2) (USD $)	12 Months Ended			6 Months Ended
	Sep. 29, 2013	Sep. 30, 2012	Oct. 02, 2011	Sep. 29, 2013
				Contract termination
Exit activities related to vacating facilities leased under long-term non-cancelable leases	'	'	'	'
Net write off of fixed assets, primarily leasehold improvements, furniture and fixtures	'	'	'	$2,700,000
Loss recorded in connection with exit activities	7,188,000	1,261,000	1,281,000	7,200,000
Write offs of prorated portions of existing deferred items previously recognized in connection with the leases	'	'	'	300,000
Costs incurred and charged to expense	$4,799,000	'	'	$4,800,000

Income_Taxes_Details

Income Taxes (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Sep. 29, 2013	Sep. 30, 2012	Oct. 02, 2011
Income (loss) before income taxes:	'	'	'
United States	$60,547	$141,035	$126,912
Foreign	-48,015	19,761	13,580
Total income before income taxes	12,532	160,796	140,492
Current:	'	'	'
Federal	11,155	46,058	30,246
State	2,705	6,949	5,948
Foreign	11,646	8,569	9,596
Total current income tax expense	25,506	61,576	45,790
Deferred:	'	'	'
Federal	-2,965	-200	6,755
State	-637	-622	1,069
Foreign	-7,866	-4,690	-6,104
Total deferred income tax expense (benefit)	-11,468	-5,512	1,720
Total income tax expense	14,038	56,064	47,510
Reconciliation of income tax expense	'	'	'
Tax at federal statutory rate	4,386	56,278	49,172
State taxes, net of federal benefit	1,316	4,932	4,376
R&E credits	-6,622	-360	-1,689
Domestic production deduction	-828	-774	-770
Tax differential on foreign earnings	-4,263	-4,444	-4,140
Corrections of prior-year errors	3,255	'	'
Goodwill and contingent consideration	11,288	-1,552	'
Stock compensation	443	80	301
Valuation allowance	4,947	2,512	'
Other	116	-608	260
Total income tax expense	$14,038	$56,064	$47,510
Reconciliation of effective income tax rate	'	'	'
Tax at federal statutory rate (as a percent)	35.00%	35.00%	35.00%
State taxes, net of federal benefit (as a percent)	10.50%	3.10%	3.10%
R&E credits (as a percent)	-52.80%	-0.20%	-1.20%
Domestic production deduction (as a percent)	-6.60%	-0.50%	-0.60%
Tax differential on foreign earnings (as a percent)	-34.00%	-2.80%	-3.00%
Corrections of prior-year errors (as a percent)	26.00%	'	'
Goodwill and contingent consideration (as a percent)	90.00%	-1.00%	'
Stock compensation (as a percent)	3.50%	0.10%	0.20%
Valuation allowance (as a percent)	39.50%	1.60%	'
Other (as a percent)	0.90%	-0.40%	0.30%
Total income tax expense (as a percent)	112.00%	34.90%	33.80%

Income_Taxes_Details_2

Income Taxes (Details 2) (USD $)	12 Months Ended
	Sep. 29, 2013	Sep. 30, 2012	Oct. 02, 2011
Deferred Tax Asset:	'	'	'
State taxes	$452,000	$975,000	'
Reserves and contingent liabilities	5,883,000	4,689,000	'
Allowance for doubtful accounts	7,345,000	2,039,000	'
Accrued liabilities	14,425,000	13,298,000	'
Stock-based compensation	10,778,000	10,980,000	'
Loss carry-forwards	9,563,000	2,926,000	'
Valuation allowance on loss carry-forwards	-7,459,000	-2,512,000	'
Total deferred tax asset	40,987,000	32,395,000	'
Deferred Tax Liability:	'	'	'
Unbilled revenue	-47,281,000	-45,417,000	'
Prepaid expense	-7,522,000	-2,251,000	'
Intangibles	-24,933,000	-21,695,000	'
Property and equipment	-9,946,000	-8,109,000	'
Total deferred tax liability	-89,682,000	-77,472,000	'
Net deferred tax liability	-48,695,000	-45,077,000	'
Undistributed earnings of foreign subsidiaries	23,500,000	'	'
Unrecognized tax benefits that would affect the effective tax rate, if recognized	24,100,000	'	'
Period during which unrecognized tax benefits would affect the effective tax rate	'12 months	'	'
Reconciliation of the beginning and ending amount of unrecognized tax benefits	'	'	'
Beginning balance	24,092,000	25,940,000	21,806,000
Additions for current year tax positions	2,661,000	6,273,000	8,007,000
Additions for prior year tax positions	4,951,000	19,000	2,554,000
Reductions for prior year tax positions	-5,818,000	-8,072,000	-6,315,000
Settlements	'	-68,000	-112,000
Ending balance	25,886,000	24,092,000	25,940,000
Amount of interest expense (net of interest income) accrued	$2,300,000	$3,100,000	'

LongTerm_Debt_Details

Long-Term Debt (Details) (USD $)	Sep. 29, 2013	Sep. 30, 2012
In Thousands, unless otherwise specified
Long-term debt	'	'
Total long-term debt	$207,749	$83,078
Less: Current portion of long-term debt	-4,311	-2,031
Long-term debt, less current portion	203,438	81,047
Credit facilities	'	'
Long-term debt	'	'
Total long-term debt	205,000	79,233
Other	'	'
Long-term debt	'	'
Total long-term debt	$2,749	$3,845

LongTerm_Debt_Details_2

Long-Term Debt (Details 2) (USD $)

Sep. 29, 2013

Sep. 30, 2012

Sep. 30, 2012

Sep. 29, 2013

Sep. 30, 2012

Sep. 29, 2013

7-May-13

Sep. 29, 2013

Sep. 27, 2013

Sep. 29, 2013

Sep. 29, 2013

Sep. 29, 2013

Sep. 29, 2013

Sep. 29, 2013

Sep. 29, 2013

Sep. 29, 2013

Sep. 29, 2013

7-May-13

Sep. 29, 2013

7-May-13

Sep. 29, 2013

7-May-13

7-May-13

Sep. 29, 2013

7-May-13

Credit facility

Bank overdraft facility

Bank overdraft facility

Standby letters of credit under letter of credit agreements

Amended Credit Agreement

Amended Credit Agreement

Amended Credit Agreement

Amended Credit Agreement

Amended Credit Agreement

Amended Credit Agreement

Amended Credit Agreement

Amended Credit Agreement

Amended Credit Agreement

Amended Credit Agreement

Term loan

Term loan

Revolving Credit Facility

Revolving Credit Facility

Standby letters of credit

Standby letters of credit

Swingline loans

Multicurrency borrowings and letter of credit

Multicurrency borrowings and letter of credit

item

item

item

Maximum

Maximum

Minimum

Base rate

Base rate

Base rate

Base rate

Eurocurrency rate

U.S. federal funds base rate

Eurocurrency base rate

Bank's prime rate

Eurocurrency base rate

Long-term debt

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Maximum borrowing capacity

'

'

$460,000,000

'

'

$40,000,000

$665,000,000

'

'

'

'

'

'

'

'

'

'

$205,000,000

'

$460,000,000

'

$200,000,000

$20,000,000

'

$150,000,000

Term of revolving credit facility

'

'

'5 years

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Amount available for borrowing under facility

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

447,600,000

'

'

'

'

'

'

Amount available for borrowing under facility without violation of debt covenants

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

265,500,000

'

'

'

'

'

'

Interest rate basis

'

'

'

'

'

'

'

'

'

'

'

'

'U.S. federal funds rate

'Eurocurrency rate

'bank's prime rate

'Eurocurrency rate

'

'

'

'

'

'

'

'

'

Basis spread on variable rate (as a percent)

'

'

'

'

'

'

'

'

'

'

'

'

0.50%

1.00%

1.00%

'

'

'

'

'

'

'

'

'

'

Margin spread on variable rate basis, low end of the range (as a percent)

'

'

'

'

'

'

'

'

'

'

'

0.15%

'

'

'

1.15%

'

'

'

'

'

'

'

'

'

Margin spread on variable rate basis, high end of the range (as a percent)

'

'

'

'

'

'

'

'

'

'

'

1.00%

'

'

'

2.00%

'

'

'

'

'

'

'

'

'

Consolidated leverage ratio

'

'

'

'

'

'

'

'

'

2.5

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Consolidated fixed charge coverage ratio

'

'

'

'

'

'

'

'

'

'

1.25

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Amount of non-recurring charges incurred in connection with corporate restructurings to be added back to Consolidated Net Income

'

'

'

'

'

'

'

'

34,000,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Amount of non-cash charges incurred in connection with the Four Programs to be added back to Consolidated Net Income

'

'

'

'

'

'

'

'

36,000,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Term of borrowings

'

'

'

'

'

'

'5 years

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Principal payment due in year 1

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

0

'

'

'

'

'

'

'

Principal payment in year 2

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

10,300,000

'

'

'

'

'

'

'

Principal payment in year 3

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

10,300,000

'

'

'

'

'

'

'

Principal payment in year 4

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

15,400,000

'

'

'

'

'

'

'

Principal payment in year 5

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

15,400,000

'

'

'

'

'

'

'

Borrowings outstanding

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

205,000,000

'

'

'

'

'

'

'

'

Letters of credit outstanding

'

'

'

'

'

6,500,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

12,400,000

'

'

6,400,000

'

Weighted-average interest rate (as a percent)

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

1.56%

'

'

'

'

'

'

'

'

Weighted average interest rate (as a percent)

'

'

'

'

'

'

'

1.84%

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Bank overdraft facility

'

'

'

900,000

500,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Number of foreign affiliates

'

'

'

1

1

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Number of agreements entered

'

'

'

'

'

3

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Number of banks with whom entity entered into agreement

'

'

'

'

'

3

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Capital leases

1,800,000

2,800,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Property and equipment loans

100,000

500,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Scheduled maturities of long-term debt

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

2014

4,311,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

2015

10,868,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

2016

15,753,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

2017

15,379,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

2018

161,438,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Total long-term debt

$207,749,000

$83,078,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Leases_Details

Leases (Details) (USD $)	12 Months Ended
	Sep. 29, 2013	Sep. 30, 2012	Oct. 02, 2011
Leases	'	'	'
Expense associated with operating leases	$80,800,000	$76,600,000	$71,900,000
Operating lease commitments	'	'	'
2014	71,293,000	'	'
2015	54,169,000	'	'
2016	35,265,000	'	'
2017	23,898,000	'	'
2018	14,771,000	'	'
Beyond	24,628,000	'	'
Total	224,024,000	'	'
Capital lease commitments	'	'	'
2014	874,000	'	'
2015	648,000	'	'
2016	384,000	'	'
2017	5,000	'	'
Total	1,911,000	'	'
Less: Amounts representing interest	-132,000	'	'
Net present value	1,779,000	'	'
Lease contract termination costs	4,500,000	1,300,000	1,300,000
Reconciliation of the beginning and ending balances of liabilities related to contract termination costs	'	'	'
Balance at the beginning of the period	2,940,000	'	'
Costs incurred and charged to expense	4,799,000	'	'
Adjustments	-1,466,000	'	'
Balance at the end of the period	6,273,000	2,940,000	'
ECS	'	'	'
Reconciliation of the beginning and ending balances of liabilities related to contract termination costs	'	'	'
Costs incurred and charged to expense	3,744,000	'	'
Adjustments	-34,000	'	'
Balance at the end of the period	3,710,000	'	'
TSS	'	'	'
Reconciliation of the beginning and ending balances of liabilities related to contract termination costs	'	'	'
Balance at the beginning of the period	2,940,000	'	'
Costs incurred and charged to expense	1,055,000	'	'
Adjustments	-1,432,000	'	'
Balance at the end of the period	$2,563,000	'	'

Stockholders_Equity_and_Stock_2

Stockholders' Equity and Stock Compensation Plans (Details) (USD $)

12 Months Ended

12 Months Ended

12 Months Ended

1 Months Ended

12 Months Ended

Sep. 29, 2013

Sep. 30, 2012

Oct. 02, 2011

Sep. 29, 2013

Sep. 29, 2013

Sep. 30, 2012

Sep. 29, 2013

Sep. 29, 2013

Sep. 29, 2013

Sep. 29, 2013

Mar. 31, 2006

Sep. 29, 2013

Sep. 29, 2013

Sep. 29, 2013

Sep. 29, 2013

Sep. 30, 2012

Oct. 02, 2011

Stock options

RSUs

RSUs

Restricted Stock

2003 Outside Director Stock Option Plan

2003 Outside Director Stock Option Plan

EIP

EIP

EIP

EIP

EIP

ESPP

ESPP

ESPP

Minimum

Stock options

Stock options

RSUs

Restricted Stock

Stockholder's equity and stock compensation plans

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Available for future awards

'

'

'

3,400,000

'

'

938,114

400,000

'

6,086,216

'

'

'

'

2,373,290

'

'

Percentage of market value on the grant date at which participant has option to purchase common stock

'

'

'

'

'

'

'

'

100.00%

'

'

'

'

'

'

'

'

Expiration period

'

'

'

'

'

'

'

'10 years

'

'

'10 years

'8 years

'

'

'

'

'

Vesting period

'

'

'

'

'4 years

'4 years

'

'

'

'

'4 years

'

'

'3 years

'

'

'

Percentage of vesting rights after specified period

'

'

'

'

'

'

'

'

'

'

'25% on the first anniversary of the grant date, and the balance vests monthly thereafter

'25% on each anniversary of the grant date

'25% on each anniversary of the grant date

'

'

'

'

Maximum amount that an employee can contribute during a purchase right period

'

'

'

'

'

'

'

'

'

'

'

'

'

'

$5,000

'

'

Exercise price as percentage of fair market value on the first day of purchase right period

'

'

'

'

'

'

'

'

'

'

'

'

'

'

100.00%

'

'

Exercise price as percentage of fair market value on the last day of purchase right period

'

'

'

'

'

'

'

'

'

'

'

'

'

'

85.00%

'

'

Stock-based compensation and related income tax benefits

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Stock-based compensation expense

8,775,000

10,839,000

10,582,000

'

'

'

'

'

'

'

'

'

'

'

800,000

900,000

1,000,000

Income tax benefit related to stock-based compensation

-3,048,000

-4,288,000

-3,804,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Stock-based compensation, net of tax benefit

$5,727,000

$6,551,000

$6,778,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Stockholders_Equity_and_Stock_3

Stockholders' Equity and Stock Compensation Plans (Details 2) (Stock options, USD $)	12 Months Ended
	Sep. 29, 2013	Sep. 30, 2012	Oct. 02, 2011
Stock options	'	'	'
Number of Options	'	'	'
Outstanding at the beginning of the year (in shares)	4,876,000	'	'
Granted (in shares)	287,000	'	'
Exercised (in shares)	-793,000	'	'
Cancelled (in shares)	-106,000	'	'
Outstanding at the end of the period (in shares)	4,264,000	4,876,000	'
Vested or expected to vest at the end of the period (in shares)	4,229,000	'	'
Exercisable at the end of the period (in shares)	3,079,000	'	'
Weighted-Average Exercise Price per Share	'	'	'
Outstanding at the beginning of the year (in dollars per share)	$21.50	'	'
Granted (in dollars per share)	$24.24	'	'
Exercised (in dollars per share)	$20.18	'	'
Cancelled (in dollars per share)	$23.72	'	'
Outstanding at the end of the period (in dollars per share)	$21.88	$21.50	'
Vested or expected to vest at the end of the period (in dollars per share)	$21.90	'	'
Exercisable at the end of the period (in dollars per share)	$21.16	'	'
Weighted-Average Remaining Contractual Term	'	'	'
Outstanding at the end of the period	'3 years 8 months 12 days	'	'
Vested or expected to vest at the end of the period	'3 years 8 months 12 days	'	'
Exercisable at the end of the period	'5 years 4 months 24 days	'	'
Aggregate Intrinsic Value	'	'	'
Outstanding at the end of the period	$17,502,000	'	'
Vested or expected to vest at the end of the period	17,268,000	'	'
Exercisable at the end of the period	14,829,000	'	'
Other disclosures	'	'	'
Unrecognized compensation cost	6,300,000	'	'
Weighted-average period to recognize the unrecognized compensation cost	'1 year 8 months 12 days	'	'
Available for future awards (in shares)	3,400,000	'	'
Weighted-average fair value of stock options granted (in dollars per share)	$8.74	$8.37	$9.08
Aggregate intrinsic value of options exercised	6,400,000	6,100,000	2,300,000
Assumptions used in the Black-Scholes option-pricing model	'	'	'
Expected stock price volatility, minimum (as a percent)	41.70%	41.90%	41.80%
Expected stock price volatility, maximum (as a percent)	42.20%	44.00%	42.70%
Risk-free rate of return, annual, minimum (as a percent)	0.60%	0.70%	1.30%
Risk-free rate of return, annual, maximum (as a percent)	1.30%	1.10%	2.10%
Net cash proceeds from the exercise of stock options	16,000,000	18,200,000	8,400,000
Income tax benefit realized from exercises of nonqualified stock options and disqualifying dispositions of qualified options	$3,700,000	$3,200,000	$1,400,000

Stockholders_Equity_and_Stock_4

Stockholders' Equity and Stock Compensation Plans (Details 3) (USD $)

12 Months Ended

3 Months Ended

12 Months Ended

Sep. 29, 2013

Sep. 30, 2012

Oct. 02, 2011

Sep. 29, 2013

Sep. 30, 2012

Oct. 02, 2011

Sep. 29, 2013

Sep. 30, 2012

Oct. 02, 2011

Sep. 29, 2013

Sep. 30, 2012

Oct. 02, 2011

Sep. 29, 2013

Sep. 29, 2013

Sep. 29, 2013

Jan. 01, 2012

Sep. 29, 2013

Sep. 30, 2012

Restricted Stock Program

Restricted Stock Program

Restricted Stock Program

Restricted Stock

Restricted Stock

Restricted Stock

Performance-based restricted stock

Performance-based restricted stock

Performance-based restricted stock

Performance-based restricted stock

Performance-based restricted stock

Time-based restricted stock

RSUs

RSUs

RSUs

Minimum

Maximum

Stockholder's equity and stock compensation plans

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Initially granted (in shares)

'

'

'

'

'

'

108,350

105,567

94,606

'

'

'

'

'

'

'

'

'

Granted to date (in shares)

'

'

'

'

'

'

308,523

'

'

298,523

'

'

'

'

10,000

'

'

'

Vesting period

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'3 years

'

'4 years

'4 years

Percentage of shares that ultimately vest depending on fiscal year earnings per share growth rates

'

'

'

'

'

'

'

'

'

'

'

'

0.00%

140.00%

'

'

'

'

Period of growth in earnings per share considered to calculate percentage of shares vested

'

'

'

'

'

'

'

'

'

'3 years

'

'

'

'

'

'

'

'

Additional shares awarded based on performance-based adjustments (in shares)

'

'

'

'

'

'

'

'

'

4,947

5,305

8,356

'

'

'

'

'

'

Base rate percentage for performance based adjustments

'

'

'

'

'

'

'

'

'

100.00%

'

'

'

'

'

'

'

'

Shares available for future award (in shares)

'

'

'

'

'

'

938,114

'

'

'

'

'

'

'

'

'

'

'

Number of Shares

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Nonvested balance at the beginning of the period (in shares)

'

'

'

'

'

'

190,000

'

'

'

'

'

'

'

'

'

172,000

'

Granted (in shares)

'

'

'

'

'

'

113,000

'

'

'

'

'

'

'

'

181,348

226,655

'

Vested (in shares)

'

'

'

'

'

'

-96,000

'

'

'

'

'

'

'

'

'

-43,000

'

Forfeited (in shares)

'

'

'

'

'

'

-4,000

'

'

'

'

'

'

'

'

'

-23,000

'

Nonvested balance at the end of the period (in shares)

'

'

'

'

'

'

203,000

190,000

'

'

'

'

'

'

'

'

333,140

172,000

Vested or expected to vest at the end of the period (in shares)

'

'

'

'

'

'

203,000

'

'

'

'

'

'

'

'

'

'

'

Weighted-Average Grant Date Fair Value

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Nonvested balance at the beginning of the period (in dollars per share)

'

'

'

'

'

'

$23.08

'

'

'

'

'

'

'

'

'

$22.53

'

Granted (in dollars per share)

'

'

'

'

'

'

$24.23

'

'

'

'

'

'

'

'

$22.53

$24.32

'

Vested (in dollars per share)

'

'

'

'

'

'

$23.45

'

'

'

'

'

'

'

'

'

$22.53

'

Forfeited (in dollars per share)

'

'

'

'

'

'

$22.85

'

'

'

'

'

'

'

'

'

$23.27

'

Nonvested balance at the end of the period (in dollars per share)

'

'

'

'

'

'

$23.55

$23.08

'

'

'

'

'

'

'

'

$23.70

$22.53

Vested or expected to vest at the end of the period (in dollars per share)

'

'

'

'

'

'

$23.55

'

'

'

'

'

'

'

'

'

'

'

Restricted Stock Program

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Stock-based compensation expense

$8,775,000

$10,839,000

$10,582,000

$2,200,000

$3,000,000

$1,700,000

'

'

'

'

'

'

'

'

'

'

'

'

Unrecognized compensation cost

'

'

'

$7,100,000

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Stockholders_Equity_and_Stock_5

Stockholders' Equity and Stock Compensation Plans (Details 4) (USD $)	12 Months Ended
Share data in Thousands, except Per Share data, unless otherwise specified	Sep. 29, 2013	Sep. 30, 2012	Oct. 02, 2011
ESPP	'	'	'
Stock-based compensation expense	$8,775,000	$10,839,000	$10,582,000
ESPP	'	'	'
Stockholder's equity and stock compensation plans	'	'	'
Shares purchased	253	289	246
Weighted-average purchase price (in dollars per share)	$21.96	$18.35	$21.30
Cash received from exercise of purchase rights	5,551,000	5,300,000	5,249,000
Aggregate intrinsic value	1,140,000	935,000	926,000
Assumptions used in the Black-Scholes option-pricing model	'	'	'
Expected stock price volatility (as a percent)	27.10%	34.70%	38.00%
Risk-free rate of return, annual (as a percent)	0.10%	0.10%	0.30%
Expected life	'1 year	'1 year	'1 year
ESPP	'	'	'
Stock-based compensation expense	800,000	900,000	1,000,000
Unrecognized compensation cost	200,000	200,000	'
Accumulated amount by participants to purchase the entity's common stock	$3,100,000	'	'

Retirement_Plans_Details

Retirement Plans (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Sep. 29, 2013	Sep. 30, 2012	Oct. 02, 2011
Retirement Plans	'	'	'
Minimum service period for employee to participate in the defined contribution plans	'1 year	'	'
Employer contributions to the plans	$9.50	$14.70	$14.10
Assets related to deferred compensation plans	17.2	13.4	'
Liabilities related to deferred compensation plans	$16.10	$12.90	'

Earnings_Per_Share_Details

Earnings Per Share (Details) (USD $)	3 Months Ended								12 Months Ended
In Thousands, except Share data, unless otherwise specified	Sep. 29, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 30, 2012	Sep. 30, 2012	Jul. 01, 2012	Apr. 01, 2012	Jan. 01, 2012	Sep. 29, 2013	Sep. 30, 2012	Oct. 02, 2011
Number of weighted-average shares used to compute basic and diluted EPS:	'	'	'	'	'	'	'	'	'	'	'
Net income (loss) attributable to Tetra Tech	$25,200	($78,385)	$24,820	$26,224	$30,432	$29,054	$22,284	$22,610	($2,141)	$104,380	$90,039
Weighted-average common shares outstanding - basic	64,272,000	64,832,000	64,551,000	63,864,000	63,623,000	63,387,000	63,072,000	62,433,000	64,544,000	63,217,000	62,053,000
Effect of diluted stock options and unvested restricted stock	'	'	'	'	'	'	'	'	'	717,000	722,000
Weighted-average common stock outstanding - diluted	64,853,000	64,832,000	65,472,000	64,608,000	64,396,000	64,179,000	63,817,000	63,068,000	64,544,000	63,934,000	62,775,000
Net income (loss) attributable to Tetra Tech per share:	'	'	'	'	'	'	'	'	'	'	'
Basic (in dollars per share)	$0.39	($1.21)	$0.38	$0.41	$0.48	$0.46	$0.35	$0.36	($0.03)	$1.65	$1.45
Diluted (in dollars per share)	$0.39	($1.21)	$0.38	$0.41	$0.47	$0.45	$0.35	$0.36	($0.03)	$1.63	$1.43
Options	'	'	'	'	'	'	'	'	'	'	'
Antidilutive securities	'	'	'	'	'	'	'	'	'	'	'
Securities excluded from the calculation of dilutive potential common shares	'	'	'	'	'	'	'	'	'	1,900,000	2,600,000
Potential common shares	'	'	'	'	'	'	'	'	'	'	'
Antidilutive securities	'	'	'	'	'	'	'	'	'	'	'
Securities excluded from the calculation of dilutive potential common shares	'	'	'	'	'	'	'	'	500,000	'	'

Derivative_Financial_Instrumen2

Derivative Financial Instruments (Details)

Sep. 29, 2013

Sep. 30, 2012

Oct. 02, 2011

Sep. 29, 2013

Mar. 31, 2013

Jul. 03, 2011

Jul. 03, 2011

Mar. 28, 2010

Mar. 28, 2010

Sep. 29, 2013

Sep. 29, 2013

Sep. 29, 2013

Sep. 29, 2013

Not designated as hedging instruments

Not designated as hedging instruments

Not designated as hedging instruments

Foreign currency forward contracts and interest rate swap agreements

Foreign currency forward contracts

Foreign currency forward contracts

Foreign currency forward contracts

Foreign currency forward contracts

Foreign currency forward contracts

Interest rate swap agreements

Interest rate swap agreement bearing fixed rate 1.36%

Interest rate swap agreement bearing fixed rate 1.34%

Interest rate swap agreement bearing fixed rate 1.35%

item

item

item

Derivatives designated as hedging instruments

USD ($)

USD ($)

CAD

USD ($)

CAD

Designated as cash flow hedges

Designated as cash flow hedges

Designated as cash flow hedges

Designated as cash flow hedges

USD ($)

item

Derivatives designated as hedging instruments

Derivatives designated as hedging instruments

Derivatives designated as hedging instruments

Derivatives designated as hedging instruments

USD ($)

USD ($)

USD ($)

USD ($)

item

Derivative financial instruments

'

'

'

'

'

'

'

'

'

'

'

'

'

Notional amount of each new foreign currency forward contract

'

'

'

'

'

$4,200,000

4,200,000

$3,900,000

4,200,000

'

'

'

'

Number of foreign currency forward contracts settled

'

'

'

'

1

'

'

'

'

'

'

'

'

Settlement amount of the foreign currency forward contract

'

'

'

'

3,900,000

'

'

'

'

'

'

'

'

Number of derivative agreements

'

'

'

'

'

'

'

'

'

3

'

'

'

Amount of effective portion of derivatives before tax effect

'

'

'

'

'

'

'

'

'

900,000

'

'

'

Amount expected to be reclassified from accumulated other comprehensive income to interest expense

'

'

'

'

'

'

'

'

'

900,000

'

'

'

Period of reclassification from accumulated other comprehensive income to interest expense

'

'

'

'

'

'

'

'

'

'12 months

'

'

'

Notional Amount

'

'

'

'

'

'

'

'

'

'

51,250,000

51,250,000

51,250,000

Fixed Rate (as a percent)

'

'

'

'

'

'

'

'

'

'

1.36%

1.34%

1.35%

Amounts excluded from effectiveness testing

'

'

'

$0

'

'

'

'

'

'

'

'

'

Number of derivative instruments

0

0

0

'

'

'

'

'

'

'

'

'

'

Derivative_Financial_Instrumen3

Derivative Financial Instruments (Details 2) (Derivatives designated as hedging instruments, USD $)	Sep. 29, 2013	Sep. 30, 2012
In Thousands, unless otherwise specified
Derivative financial instruments	'	'
Derivative liabilities, Fair Value of Derivative Instruments	$987	$348
Foreign currency forward contracts | Other current liabilities	'	'
Derivative financial instruments	'	'
Derivative liabilities, Fair Value of Derivative Instruments	'	348
Interest rate swap agreements | Designated as cash flow hedges | Other current liabilities	'	'
Derivative financial instruments	'	'
Derivative liabilities, Fair Value of Derivative Instruments	$987	'

Fair_Value_Measurements_Detail

Fair Value Measurements (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Sep. 29, 2013
Fair Value Measurements	'
Net borrowing under amended credit agreement	$205

Joint_Ventures_Details

Joint Ventures (Details) (USD $)	3 Months Ended								12 Months Ended
	Sep. 29, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 30, 2012	Sep. 30, 2012	Jul. 01, 2012	Apr. 01, 2012	Jan. 01, 2012	Sep. 29, 2013	Sep. 30, 2012	Oct. 02, 2011
Aggregate revenue of consolidated joint ventures	$698,376,000	$614,835,000	$641,999,000	$658,545,000	$719,405,000	$684,698,000	$624,345,000	$682,627,000	$2,613,755,000	$2,711,075,000	$2,573,144,000
Restricted cash and cash equivalents of consolidated joint ventures	4,500,000	'	'	'	5,000,000	'	'	'	4,500,000	5,000,000	'
Unconsolidated Joint Ventures	'	'	'	'	'	'	'	'	'	'	'
Equity in earnings from unconsolidated joint ventures	'	'	'	'	'	'	'	'	3,461,000	2,916,000	4,877,000
Carrying value of assets of unconsolidated joint ventures	24,000,000	'	'	'	19,000,000	'	'	'	24,000,000	19,000,000	'
Carrying value of liabilities of unconsolidated joint ventures	21,800,000	'	'	'	15,700,000	'	'	'	21,800,000	15,700,000	'
Consolidated Joint Ventures	'	'	'	'	'	'	'	'	'	'	'
Aggregate revenue of consolidated joint ventures	'	'	'	'	'	'	'	'	15,600,000	19,300,000	74,300,000
Restricted cash and cash equivalents of consolidated joint ventures	$1,200,000	'	'	'	$1,600,000	'	'	'	$1,200,000	$1,600,000	'

Commitments_and_Contingencies_

Commitments and Contingencies (Details)	Mar. 31, 2013	Apr. 17, 2012	Jun. 28, 2013
	BPR Triax	BPR Triax	Violation of Section 10(b) and 20(a) of the Securities Exchange Act of 1934 and related rules thereof
	item	employee	item
Loss contingencies	'	'	'
Number of employees of BPR Triax charged with allegations of corruption	'	2	'
Number of government contracts cited in testimony	5	'	'
Number of officers against whom complaint was filed	'	'	2

Reportable_Segments_Details

Reportable Segments (Details) (USD $)	3 Months Ended								12 Months Ended
	Sep. 29, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 30, 2012	Sep. 30, 2012	Jul. 01, 2012	Apr. 01, 2012	Jan. 01, 2012	Sep. 29, 2013	Sep. 30, 2012	Oct. 02, 2011
Financial information concerning reportable segments	'	'	'	'	'	'	'	'	'	'	'
Revenue	$698,376,000	$614,835,000	$641,999,000	$658,545,000	$719,405,000	$684,698,000	$624,345,000	$682,627,000	$2,613,755,000	$2,711,075,000	$2,573,144,000
Operating income	40,626,000	-99,884,000	37,667,000	41,809,000	48,470,000	46,261,000	35,543,000	36,093,000	20,218,000	166,367,000	146,422,000
Depreciation	'	'	'	'	'	'	'	'	29,548,000	26,651,000	27,137,000
Goodwill impairment charge	'	56,600,000	'	'	'	'	'	'	56,600,000	914,000	'
Amortization expense for intangible assets	'	'	'	'	'	'	'	'	32,400,000	29,600,000	28,000,000
Fair value adjustments to contingent consideration liabilities	81,800,000	'	'	'	'	'	'	'	81,800,000	'	'
Total assets	1,799,092,000	'	'	'	1,671,030,000	'	'	'	1,799,092,000	1,671,030,000	'
ECS	'	'	'	'	'	'	'	'	'	'	'
Financial information concerning reportable segments	'	'	'	'	'	'	'	'	'	'	'
Revenue	'	'	'	'	'	'	'	'	1,035,983,000	1,155,256,000	1,110,060,000
Operating income	'	'	'	'	'	'	'	'	44,598,000	96,220,000	99,868,000
Depreciation	'	'	'	'	'	'	'	'	10,494,000	10,126,000	12,067,000
Goodwill impairment charge	'	'	'	'	'	'	'	'	56,600,000	914,000	'
Total assets	912,996,000	'	'	'	915,571,000	'	'	'	912,996,000	915,571,000	'
TSS	'	'	'	'	'	'	'	'	'	'	'
Financial information concerning reportable segments	'	'	'	'	'	'	'	'	'	'	'
Revenue	'	'	'	'	'	'	'	'	932,375,000	1,020,779,000	995,249,000
Operating income	'	'	'	'	'	'	'	'	71,842,000	71,767,000	69,977,000
Depreciation	'	'	'	'	'	'	'	'	2,839,000	3,227,000	2,028,000
Total assets	673,864,000	'	'	'	638,405,000	'	'	'	673,864,000	638,405,000	'
RCM	'	'	'	'	'	'	'	'	'	'	'
Financial information concerning reportable segments	'	'	'	'	'	'	'	'	'	'	'
Revenue	'	'	'	'	'	'	'	'	725,689,000	621,957,000	604,651,000
Operating income	'	'	'	'	'	'	'	'	-6,706,000	22,374,000	13,183,000
Depreciation	'	'	'	'	'	'	'	'	13,160,000	10,233,000	10,101,000
Total assets	435,053,000	'	'	'	311,051,000	'	'	'	435,053,000	311,051,000	'
Inter-segment Elimination	'	'	'	'	'	'	'	'	'	'	'
Financial information concerning reportable segments	'	'	'	'	'	'	'	'	'	'	'
Revenue	'	'	'	'	'	'	'	'	-80,292,000	-86,917,000	-136,816,000
Corporate	'	'	'	'	'	'	'	'	'	'	'
Financial information concerning reportable segments	'	'	'	'	'	'	'	'	'	'	'
Operating income	'	'	'	'	'	'	'	'	-89,516,000	-23,994,000	-36,606,000
Depreciation	'	'	'	'	'	'	'	'	3,055,000	3,065,000	2,941,000
Goodwill impairment charge	'	'	'	'	'	'	'	'	56,600,000	'	'
Amortization expense for intangible assets	'	'	'	'	'	'	'	'	32,400,000	29,600,000	28,000,000
Fair value adjustments to contingent consideration liabilities	9,600,000	'	'	'	19,200,000	'	'	'	9,600,000	19,200,000	1,800,000
Corporate	'	'	'	'	'	'	'	'	'	'	'
Financial information concerning reportable segments	'	'	'	'	'	'	'	'	'	'	'
Total assets	($222,821,000)	'	'	'	($193,997,000)	'	'	'	($222,821,000)	($193,997,000)	'

Reportable_Segments_Details_2

Reportable Segments (Details 2) (USD $)	3 Months Ended								12 Months Ended
In Thousands, unless otherwise specified	Sep. 29, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 30, 2012	Sep. 30, 2012	Jul. 01, 2012	Apr. 01, 2012	Jan. 01, 2012	Sep. 29, 2013	Sep. 30, 2012	Oct. 02, 2011
Reportable Segments	'	'	'	'	'	'	'	'	'	'	'
Revenue	$698,376	$614,835	$641,999	$658,545	$719,405	$684,698	$624,345	$682,627	$2,613,755	$2,711,075	$2,573,144
United States	'	'	'	'	'	'	'	'	'	'	'
Reportable Segments	'	'	'	'	'	'	'	'	'	'	'
Revenue	'	'	'	'	'	'	'	'	1,915,780	2,046,700	1,976,452
Long-Lived Assets	110,313	'	'	'	100,958	'	'	'	110,313	100,958	102,316
Foreign countries	'	'	'	'	'	'	'	'	'	'	'
Reportable Segments	'	'	'	'	'	'	'	'	'	'	'
Revenue	'	'	'	'	'	'	'	'	697,975	664,375	596,692
Long-Lived Assets	$90,435	'	'	'	$70,010	'	'	'	$90,435	$70,010	$78,198

Reportable_Segments_Details_3

Reportable Segments (Details 3) (USD $)	3 Months Ended								12 Months Ended
In Thousands, unless otherwise specified	Sep. 29, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 30, 2012	Sep. 30, 2012	Jul. 01, 2012	Apr. 01, 2012	Jan. 01, 2012	Sep. 29, 2013	Sep. 30, 2012	Oct. 02, 2011
									item	item
Reportable Segments	'	'	'	'	'	'	'	'	'	'	'
Number of clients exceeding threshold	'	'	'	'	'	'	'	'	0	0	'
Revenue by client sector	'	'	'	'	'	'	'	'	'	'	'
Threshold percentage for disclosure of revenue from a single client	'	'	'	'	'	'	'	'	10.00%	'	'
Revenue	$698,376	$614,835	$641,999	$658,545	$719,405	$684,698	$624,345	$682,627	$2,613,755	$2,711,075	$2,573,144
International	'	'	'	'	'	'	'	'	'	'	'
Revenue by client sector	'	'	'	'	'	'	'	'	'	'	'
Revenue	'	'	'	'	'	'	'	'	697,975	664,375	596,692
U.S. commercial	'	'	'	'	'	'	'	'	'	'	'
Revenue by client sector	'	'	'	'	'	'	'	'	'	'	'
Revenue	'	'	'	'	'	'	'	'	693,677	718,457	577,782
U.S. federal government	'	'	'	'	'	'	'	'	'	'	'
Revenue by client sector	'	'	'	'	'	'	'	'	'	'	'
Revenue	'	'	'	'	'	'	'	'	829,790	1,008,424	1,115,729
U.S. state and local government	'	'	'	'	'	'	'	'	'	'	'
Revenue by client sector	'	'	'	'	'	'	'	'	'	'	'
Revenue	'	'	'	'	'	'	'	'	$392,313	$319,819	$282,941

Quarterly_Financial_Informatio2

Quarterly Financial Information-Unaudited (Details) (USD $)	3 Months Ended								12 Months Ended
Share data in Thousands, except Per Share data, unless otherwise specified	Sep. 29, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 30, 2012	Sep. 30, 2012	Jul. 01, 2012	Apr. 01, 2012	Jan. 01, 2012	Sep. 29, 2013	Sep. 30, 2012	Oct. 02, 2011
		item
Quarterly financial information	'	'	'	'	'	'	'	'	'	'	'
Number of programs for which project charges recorded	'	4	'	'	'	'	'	'	'	'	'
Reduction in operating income due to project charge	'	$35,500,000	'	'	'	'	'	'	'	'	'
Reduction in operating income due to charge related to Eastern Canada and global mining operations	'	28,200,000	'	'	'	'	'	'	'	'	'
Operating Income (loss)	40,626,000	-99,884,000	37,667,000	41,809,000	48,470,000	46,261,000	35,543,000	36,093,000	20,218,000	166,367,000	146,422,000
Non-cash goodwill impairment charges	'	56,600,000	'	'	'	'	'	'	56,600,000	914,000	'
Revenue	698,376,000	614,835,000	641,999,000	658,545,000	719,405,000	684,698,000	624,345,000	682,627,000	2,613,755,000	2,711,075,000	2,573,144,000
Net income (loss) attributable to Tetra Tech	25,200,000	-78,385,000	24,820,000	26,224,000	30,432,000	29,054,000	22,284,000	22,610,000	-2,141,000	104,380,000	90,039,000
Net income (loss) attributable to Tetra Tech per share:	'	'	'	'	'	'	'	'	'	'	'
Basic (in dollars per share)	$0.39	($1.21)	$0.38	$0.41	$0.48	$0.46	$0.35	$0.36	($0.03)	$1.65	$1.45
Diluted (in dollars per share)	$0.39	($1.21)	$0.38	$0.41	$0.47	$0.45	$0.35	$0.36	($0.03)	$1.63	$1.43
Weighted-average common shares outstanding:	'	'	'	'	'	'	'	'	'	'	'
Basic (in shares)	64,272	64,832	64,551	63,864	63,623	63,387	63,072	62,433	64,544	63,217	62,053
Diluted (in shares)	64,853	64,832	65,472	64,608	64,396	64,179	63,817	63,068	64,544	63,934	62,775
RCM	'	'	'	'	'	'	'	'	'	'	'
Quarterly financial information	'	'	'	'	'	'	'	'	'	'	'
Number of programs for which project charges recorded	'	3	'	'	'	'	'	'	'	'	'
Operating Income (loss)	'	'	'	'	'	'	'	'	-6,706,000	22,374,000	13,183,000
Revenue	'	'	'	'	'	'	'	'	$725,689,000	$621,957,000	$604,651,000

SCHEDULE_IIVALUATION_AND_QUALI1

SCHEDULE II-VALUATION AND QUALIFYING ACCOUNTS AND RESERVES (Details) (USD $)	12 Months Ended
In Thousands, unless otherwise specified	Sep. 29, 2013	Sep. 30, 2012	Oct. 02, 2011
Allowance for doubtful accounts	'	'	'
Changes in valuation and qualifying accounts and reserves	'	'	'
Balance at Beginning of Period	$35,552	$32,244	$32,926
Additions (Charged to Costs, Expenses and Revenue)	13,818	4,768	3,733
Deductions	-4,452	-2,356	-6,478
Other	-295	896	2,063
Balance at End of Period	44,623	35,552	32,244
Income tax valuation allowance	'	'	'
Changes in valuation and qualifying accounts and reserves	'	'	'
Balance at Beginning of Period	2,512	'	5,526
Additions (Charged to Costs, Expenses and Revenue)	4,947	2,512	'
Other	'	'	-5,526
Balance at End of Period	$7,459	$2,512	'