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                                                                     EXHIBIT 5.1


                    PAUL, WEISS, RIFKIND, WHARTON & GARRISON
                           1285 AVENUE OF THE AMERICAS
                          NEW YORK, NEW YORK 10019-6064
                                 (212) 373-3000




                                                     August 9, 1999




Anteon Corporation
3211 Jermantown Road
Suite 700
Fairfax, VA 22030-2801

                  REGISTRATION STATEMENT ON FORM S-4

Ladies and Gentlemen:

                  In connection with the referenced Registration Statement on
Form S-4 (the "Registration Statement") filed today by Anteon Corporation (the
"Issuer"), a Virginia corporation, with the Securities and Exchange Commission
(the "SEC") under the Securities Act of 1933, as amended (the "Act"), and the
rules and regulations under the Act (the "Rules"), we have been requested to
render our opinion as to the legality of the securities being registered. The
Registration Statement registers under the Act the issuance of the Issuer's 12%
Senior Subordinated Notes due 2009 (the "Exchange Notes"). The Exchange Notes
are to be offered in exchange for the Issuer's outstanding 12% Senior
Subordinated Notes due 2009 (the "Initial Notes"), and will be issued under the
Indenture (the "Indenture"), dated as of May 11, 1999, between the Issuer,
Techmatics, Inc., Vector Data Systems Inc., and IBJ Whitehall Bank & Trust
Company, as trustee (the "Trustee").





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Anteon Corporation
August 9, 1999
Page -2-



Capitalized terms used and not otherwise defined in this letter have the
respective meanings given those terms in the Registration Statement.

                  In connection with this opinion, we have examined originals,
conformed copies or photocopies, certified or otherwise identified to our
satisfaction, of the following documents (collectively, the "Documents"):

                  (i)      the Registration Statement (including its exhibits);

                  (ii)     the Indenture included as Exhibit 4.1 to the
                           Registration Statement;

                  (iii)    the proposed form of the Exchange Notes included as
                           Exhibit A to the Indenture; and

                  (iv)     the Registration Rights Agreement, dated as of May 6,
                           1999, among the Issuer, Vector Data Systems, Inc.,
                           Techmatics, Inc., Credit Suisse First Boston
                           Corporation, Deutsche Bank Securities Inc. and Legg
                           Mason Wood Walker, Incorporated (the "Registration
                           Rights Agreement").

In addition, we have examined: (i) those corporate records of the Issuer that we
have considered appropriate; and (ii) those other certificates, agreements and
documents that we deemed relevant and necessary as a basis for the opinion
expressed below.

                  In our examination of the documents and in rendering the
opinion set forth below, we have assumed, without independent investigation, (i)
the genuineness of all signatures, the authenticity of all documents submitted
to us as originals, the conformity of the original documents of all documents
submitted to us as certified,





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Anteon Corporation
August 9, 1999
Page -3-



photostatic, reproduced or conformed copies of validly existing agreements or
other documents, the authenticity of all the latter documents and the legal
capacity of all individuals who have executed any of the documents which we
examined, (ii) that the Exchange Notes will be issued as described in the
Registration Statement, (iii) that the Indenture was duly authorized, executed
and delivered by the Trustee and is a valid and binding agreement of the
Trustee, (iv) that the Registration Rights Agreement was duly authorized,
executed and delivered by the Initial Purchasers and is a valid and binding
agreement of the Initial Purchasers and (v) that the Exchange Notes will be in
substantially the form attached to the Indenture and that any information
omitted from them will be properly added. We have also relied upon certificates
of public officials and officers of the Issuer.

                  Based on the above, and subject to the stated assumptions,
exceptions and qualifications, we are of the opinion that, when the Exchange
Notes are duly issued, authenticated and delivered in accordance with the terms
of the Indenture and the Registration Rights Agreement, the Exchange Notes will
be legal, valid and binding obligations of the Issuer enforceable against it in
accordance with their terms.

                  Our opinion is subject to the qualification that the
enforceability of the Indenture and the Exchange Notes may be (i) subject to
bankruptcy, insolvency, fraudulent conveyance or transfer, reorganization,
moratorium and other similar laws affecting creditors' rights generally and (ii)
general principles of equity (regardless of whether enforcement is considered in
a proceeding at law or in equity).





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Anteon Corporation
August 9, 1999
Page -4-


                  Our opinion is limited to matters of New York law. Our opinion
is rendered only with respect to the laws, and the rules, regulations and orders
under them, which are currently in effect.

                  We hereby consent to the use of this opinion as an exhibit to
the Registration Statement and to the reference to us under the heading "Legal
Matters" in the prospectus included contained in the Registration Statement. In
giving this consent, we do not admit that we come within the category of persons
whose consent is required by the Act or the Rules.

                                Very truly yours,

                                    /s/ Paul, Weiss, Rifkind, Wharton & Garrison
                                    PAUL, WEISS, RIFKIND, WHARTON & GARRISON









