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                                                                 Exhibit 5.1


                         [HAZEL & THOMAS LETTERHEAD]


                                  July 2, 1997




Board of Directors
Deltek Systems, Inc.
8280 Greensboro Drive
McLean, Virginia  22102

                 Re:      Registration Statement on Form S-8

Gentlemen:

         We have acted as counsel to Deltek Systems, Inc., a Virginia
corporation (the "Company"), in connection with the preparation and filing by
the Company of  its registration statement on Form S-8 (the "Registration
Statement") under the Securities Act of 1933, as amended, relating to a maximum
of 900,000 shares of Common Stock, $.001 par value (the "Common Stock"), issued
or issuable upon exercise of options granted or to be granted pursuant to the
Deltek Systems, Inc., 1996 Stock Option Plan (the "1996 Plan") and relating to
an indeterminate amount of interests (and options deemed granted to
participants) in the Deltek Systems, Inc.  1996 Employee Stock Purchase Plan
(the "Purchase Plan") and relating to a maximum of 399,999 shares (exclusive of
shares purchased by the Company in the open market for issuance under the
Purchase Plan) of Common Stock issued or issuable upon exercise of options
deemed to have been granted or to be granted pursuant to the Purchase Plan.

         We have examined the 1996 Plan, the Purchase Plan, the Articles of
Incorporation, as amended, the By-Laws of the Company, the minutes of the
various meetings and consents of the Company's Board of Directors, originals or
copies of such records of the Company, agreements, certificates of public
officials, certificates of officers and representatives of the Company and
others, and such other documents, certificates, records, authorizations,
proceedings, statutes and judicial decisions as we have deemed necessary to
form the basis of the opinion expressed below.  In such examination, we have
assumed the genuineness of all signatures, the authenticity of all documents
submitted to us as originals and the conformity to originals of all documents
submitted to us as copies thereof.  As to various questions of fact material to
such opinion, we have relied upon statements and certificates of officers and
representatives of the Company and others.

         Based upon the foregoing, we are of the opinion that the 900,000
shares of Common Stock issued or issuable under the 1996 Plan and the interests
(and options deemed to be granted to participants) in the Purchase Plan to
purchase up to 399,999 shares of Common Stock and up to 399,999 shares of
Common Stock (exclusive of shares purchased by the Company in the open market
for issuance under the Purchase Plan) issued or issuable under the Purchase
Plan, all of which are subject of the Registration Statement, have 

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Board of Directors
Deltek Systems, Inc.
July 2, 1997
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been duly authorized and when issued and paid for in accordance with the 1996
Plan or the Purchase Plan, respectively, will be duly authorized, fully paid
and nonassessable.

                 We hereby consent to the use of this opinion as an exhibit to
the Registration Statement.

                                        Yours very truly,

                                        /s/ HAZEL & THOMAS, P.C.

                                        Hazel & Thomas, P.C.