Exhibit 5.1

                               December 16, 2002

Board of Directors
ManTech International Corporation
12015 Lee Jackson Highway
Fairfax, Virginia  22033-3300

Re:      ManTech International Corporation
         Registration Statement on Form S-1
         File No. 333-101226

Ladies and Gentlemen:

         We have examined the Registration Statement on Form S-1, File No.
333-101226, as amended (the "Registration Statement"), of ManTech International
Corporation, a Delaware corporation (the "Company"), filed with the Securities
and Exchange Commission (the "Commission") pursuant to the Securities Act of
1933, as amended (the "Securities Act"), in connection with the public offering
of up to 7,072,500 shares (including shares to be issued upon exercise of the
underwriters' over-allotment options) (the "Shares") of the Company's Class A
Common Stock, par value $0.01 per share, proposed to be offered by the Company
and the Selling Stockholders identified in the Registration Statement.

         In connection with rendering the opinions set forth in this letter, we
have examined and relied upon the Registration Statement, the Company's
Certificate of Incorporation and Bylaws as in effect on the date of this
opinion, resolutions of the Board of Directors of the Company, and the originals
or copies of such records, documents, certificates, memoranda and other
instruments as in our judgment are necessary or appropriate to enable us to
render the opinions expressed below.

         The opinions set forth herein are subject to the following
qualifications, which are in addition to any other qualifications contained
herein:

         (i) We have assumed without verification the genuineness of all
signatures on all documents, the legal capacity of all natural persons, the
authority of the parties (other than the Company) executing such documents, the
authenticity of all documents submitted to us as originals, and the conformity
to original documents of all documents submitted to us as copies.



ManTech International Corporation
December 16, 2002
Page 2


         (ii) The opinions set forth herein are based on existing laws,
ordinances, rules, regulations, and judicial and administrative decisions as
they presently have been interpreted, and we can give no assurance that our
opinions would not be different after any change in any of the foregoing
occurring after the date hereof.

         (iii) We have assumed without verification that, with respect to the
minutes of any meetings of the Board of Directors or any committees thereof of
the Company that we have examined, due notice of the meetings was given or duly
waived, the minutes accurately and completely reflect all actions taken at the
meetings and a quorum was present and acting throughout the meetings.

         (iv) We have assumed without verification the accuracy and completeness
of all corporate records made available to us by the Company.

         (v) We express no opinion as to the effect or application of any laws
or regulations other than the internal laws of the State of Delaware. As to
matters governed by the laws specified in the preceding sentence, we have relied
exclusively on the latest standard compilations of such statutes and laws as
reproduced in commonly accepted unofficial publications available to us.

         Based on the foregoing, upon the assumption that there will be no
material changes in the documents we have examined and the matters investigated
referred to above, we are of the opinion that (i) the Shares to be sold by the
Selling Stockholders are validly issued, fully paid and non-assessable, and (ii)
the Shares to be sold by the Company, when issued against payment therefor in
connection with the offering, will be validly issued, fully paid and
non-assessable.

         This letter does not address any matters other than those expressly
addressed herein. This letter is given for your sole benefit and use. No one
else is entitled to rely hereupon. This letter speaks only as of the date
hereof. We undertake no responsibility to update or supplement it after such
date.

         We hereby consent to your filing of this opinion as an exhibit to the
Registration Statement and to reference to our firm under the caption "Validity
of Common Stock" contained in the Prospectus included therein. In giving such
consent, we do not thereby admit that we are in the category of persons whose
consent is required under Section 7 of the Securities Act.



ManTech International Corporation
December 16, 2002
Page 3

                                                     Very truly yours,



                                                     /s/Arnold & Porter
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                                                     Arnold & Porter