Tetra Tech (TTEK) 10-K 2006 FY Annual report

EXHIBIT 23

CONSENT OF INDEPENDENT REGISTERED PUBLIC ACCOUNTING FIRM

We hereby consent to the incorporation by reference in the Registration Statements on Form S-8 (Nos. 033-46240, 033-47533, 033-80606, 033-94706, 333-11757, 333-53036 and 333-85558) of Tetra Tech, Inc. of our report dated December 27, 2006, relating to the financial statements, management’s assessment of the effectiveness of internal control over financial reporting and the effectiveness of internal control over financial reporting, which appears in the Annual Report to Stockholders, which is incorporated in this Annual Report on Form 10-K. We also consent to the incorporation by reference of our report dated December 27, 2006 relating to the financial statement schedule, which appears in this Form 10-K.

/s/ PRICEWATERHOUSECOOPERS LLP

Los Angeles, California

December 28, 2006