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                                                                     Exhibit 5.1

                                August 16, 1999

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 an aggregate of 2,100,000 shares
(of which 900,000 shares were previously registered and which registration
statement is incorporated by reference in the Registration Statement) 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").

        We have examined the 1996 Plan as amended, 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 additional
1,200,000 shares of Common Stock issued or issuable under the 1996 Plan which
are subject of the Registration Statement have been duly authorized and, when
issued and paid for in accordance with the 1996 Plan, 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.