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                                                                    EXHIBIT 10.6

                               RETENTION AGREEMENT
                               -------------------

         THIS AGREEMENT (the "Agreement") is made as of January 1, 2002 (the
"Effective Date"), by and between ManTech International Corporation, a Delaware
corporation that is the successor to a New Jersey corporation of the same name
("ManTech"), and George J. Pedersen (the "Executive").

                                   WITNESSETH
                                   ----------

         WHEREAS, ManTech has grown and its business progressed during the time
that the Executive has served as the Chairman of the Board, Chief Executive
Officer and President and also holds various other positions (whether as an
employee, board member or otherwise) of ManTech and certain of its subsidiaries,
and ManTech wishes to continue to retain and continue to employ the Executive in
such capacities;

         NOW THEREFORE, in consideration of the mutual covenants and promises
hereinafter set forth and for other good and valuable consideration, the receipt
and sufficiency of which are hereby acknowledged, ManTech and the Executive
hereby agree as follows:

         1.  Employment Duties.
             -----------------

         (a) This Agreement shall become effective on the Effective Date noted
above. During the term of this Agreement, the Executive hereby agrees to
continue to serve as Chairman of the Board, Chief Executive Officer and
President of ManTech and various other positions (whether as an employee, board
member or otherwise) of certain subsidiaries of ManTech, and ManTech hereby
agrees to employ the Executive as such. Executive shall hold such further
positions with ManTech and with any of its subsidiaries or affiliates as ManTech
shall reasonably specify from time to time. The Executive shall report to the
Board of Directors of ManTech.

         (b) Executive shall terminate the Executive Employment Agreement
between Executive and GSE Systems, Inc., an affiliate of ManTech, on or before
January 25, 2002; provided, however, ManTech and Executive agree that Executive
shall continue to serve as a member of the Board of Directors and Chairman of
the Executive Committee of the Board of Directors of GSE Systems, Inc.

         2.  Compensation.
             ------------

         (a) During the Employment Period, ManTech shall pay the Executive a
cash base salary of $1,000,000 for his service during calendar year 2002 (the
"Base Salary"). The Base Salary shall be paid to the Executive, less applicable
withholdings, in installments pursuant to ManTech's normal and customary
executive officer payroll procedures. Executive shall not be eligible for
payment of a bonus for service during calendar year 2002, and shall not be
granted employee stock options during calendar year 2002. For service following
2002, Executive's Base Salary shall be subject to change and Executive shall be
eligible for bonus and stock option grants as shall be determined

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by the Compensation Committee of the Board of Directors in its sole discretion.
Notwithstanding the foregoing, Executive shall be entitled to receive a bonus
for services performed during calendar year 2001.

         (b) In addition, during the Employment Period, ManTech shall provide
certain non-cash compensation to Executive consistent with the position of
Chairman of the Board, Chief Executive Officer and President, including without
limitation, those benefits set forth on Attachment A hereto.

         (c) In addition to the Base Salary, during the Employment Period
Executive shall be entitled to participate in such bonus and other incentive
award programs as determined by the Compensation Committee of ManTech's Board
of Directors in its sole discretion, except as provided in section 2(a) with
respect to 2002. In addition, the Executive shall be entitled to participate in
any employee benefit plans and programs as are generally applicable to executive
officers and, to the extent permitted by law, to employees of ManTech and in
such other benefit plans and programs as determined by the Compensation
Committee of ManTech's Board of Directors in its sole discretion.

         3.  Employment Period. The Executive shall be an at-will employee of
             -----------------
ManTech. The Employment Period shall commence on the Effective Date and shall
terminate on the earliest of (a) the day set forth in a writing delivered by
ManTech or the Executive in accordance with Section 4 hereof, (b) the
Executive's death, or (c) the Executive's Termination for Cause.

         4.  Termination Procedure. ManTech or Executive may terminate this
             ---------------------
Agreement at any time during the Employment Period (other than due to the
Executive's death or a termination by ManTech for Cause) if notice of such
termination is communicated by written "Notice of Termination" to the Executive
or ManTech no later than sixty (60) days prior to the desired date of
termination of this Agreement.

         5.  Termination Payments.
             --------------------

         (a) Upon the Executive's termination of employment for any reason,
ManTech shall pay to the Executive any unpaid Base Salary then in effect accrued
up to the date of termination of employment and any amount payable for accrued
but unused vacation time up to the date of termination. Other than the accrued
salary and vacation pay referenced in the preceding sentence, the Executive
shall not be entitled to any further payments or benefits, unless otherwise
agreed to in writing between ManTech and the Executive.

         (b) Notwithstanding Section 5(a), if the Executive's employment is
terminated by ManTech without Cause, ManTech also shall pay to the Executive an
amount equal to one (1) year's Base Salary at the rate in effect immediately
prior to such termination of employment, such payment to be made in a lump sum
as soon as practicable following such termination. The one (1) year period for
which the payment of severance is calculated pursuant to the preceding sentence
shall be known as the "Severance Period."

         (c) For purposes of this Agreement, "Cause" shall mean a termination of
the Executive's employment by ManTech for a reason other than: (i) a material
violation by the Executive of this Agreement which the Executive fails to cure
to ManTech's

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reasonable satisfaction within thirty (30) days after ManTech delivers to the
Executive a written notice that specifically identifies such violation; (ii) the
willful failure by the Executive to act in a manner consistent with Executive's
responsibilities or with the best interests of ManTech, after ManTech delivers
to the Executive a written demand for satisfactory performance that specifically
identifies the manner in which ManTech believes that the Executive has not
satisfactorily performed the Executive's duties and the Executive fails to cure
the existing problem to ManTech's reasonable satisfaction within thirty (30)
days; or (iii) the conviction of the Executive of a felony (other than an
offense related to the operation of an automobile which results only in a fine,
license suspension or other non-custodial penalty) or other serious crime
involving moral turpitude.

         (d) This Agreement shall not be construed to be in lieu of or to the
exclusion of any other rights, benefits and privileges to which Executive may be
entitled as an executive of ManTech or any of its subsidiaries or affiliates
under any retirement, pension, profit-sharing, insurance, hospitalization or
other plans or benefits which may now be in effect or which may hereafter be
adopted.

         6.  Confidentiality, Non-Competition and Non-Solicitation. For good and
             -----------------------------------------------------
valuable consideration, the receipt and sufficiency of which Executive hereby
acknowledges, the Executive hereby agrees as follows:

         (a) That both during the entire term of Executive's employment with
ManTech and/or any of subsidiaries and affiliates (collectively, the "Employer")
and thereafter, Executive will not publish or otherwise disclose to persons
other than those employed by Employer, without specific permission from
Employer, any Employer proprietary or confidential information which Executive
learns or acquires during the course of employment with or as a result of
performing services with Employer, and will not use such information in any way
which might be detrimental to the interests of the Employer. For purposes of
this Agreement, proprietary or confidential information includes, but is not
limited to:

             (i)  All information not generally known to the public or within
the federal, state or local government market(s) or the commercial market(s) in
which the Employer offers or provides its services, solutions or products,
pertaining to the Employer's marketing, bidding or cost plans, strategies,
forecasts or projections; practices, procedures, policies, goals or objectives
pertaining to the foregoing; contract proposals, contract bids which have been
prepared or submitted or which are proposed to be prepared or submitted, or
bidding and pricing techniques; information on Employer's cost structure;
quoting and pricing practices, procedures and policies; customer data including
customer list, contracts, contacts, representatives, requirements and needs,
specifications, data provided by or about prospective customers; supplier
information, including joint venture and subcontractor proposals; employee and
consultants' identities, skills, resumes, records and lists; and the physical
embodiments of any of the foregoing information.

             (ii) All information concerning or relating to the way the Employer
conducts their business which is not generally known to the public or within

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the federal, state or local government market(s) or the commercial market(s) in
which the Employer offers or provides its services, solutions or products (such
as Employer contracts, internal business procedures, controls, plans, licensing
techniques and practices, supplier, subcontractor and prime contractor names and
contacts and other vendor information, Employer processes, techniques, data,
computer system passwords and other computer security controls, financial
information, and distributor information) and the physical embodiments of such
information (such as check lists, samples, service and operational manuals,
contracts, proposals, printouts, correspondence, forms, listings, ledgers,
financial statements, financial reports, financial and operational analyses,
financial and operational studies, management reports of every kind, databases,
and any other written or machine-readable expression of such information as are
filed in any tangible media).

             (iii) All information not generally known to the public or within
the federal, state or local government market(s) or the commercial market(s) in
which the Employer offers or provides its services, solutions or products
concerning development of new products, services or solutions, negotiations for
new business ventures or acquisitions, future business or acquisition plans, and
similar information and the physical embodiments of such information.

             (iv)  Information which is not a public record and is not generally
known to the public or within the federal, state or local government market(s)
or the commercial market(s) in which the Employer offers or provides its
services, solutions or products regarding litigation and potential litigation
matters and the physical embodiments of such information.

             (v)   Any information which (i) is not generally known to the
-public or within the federal, state or local government market(s) or the
commercial market(s) in which the Employer offers or provides its services,
solutions or products, (ii) gives the Employer a significant advantage over its
or their competitors, or (iii) has significant economic value or potentially
significant economic value to the Employer, including the physical embodiments
of such information.

         (b) That both during the entire term of Executive's employment with
Employer and thereafter through the Severance Period, the Executive shall not:

             (i)   directly or indirectly, own, manage, operate, control or
participate in the ownership, management, operation or control of, or be
connected as an officer, employee, consultant, partner, director or otherwise
with, or have any financial interest in, or aid or assist anyone else in the
conduct of any business which competes with any services, solutions or products
conducted, offered or provided by the Employer (any such service, solution or
product, an "Employer Operation"), to any federal, state or local government
market(s) or the commercial market(s) if such Employer Operation is being
conducted or developed at any time during the term of Executive's employment
with Employer and at the later time in question;

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             (ii)  directly or indirectly, solicit any customer or any former or
prospective customer of the Employer with a view to inducing such customer to
enter into an agreement, or otherwise do business, involving an Employer
Operation with any competitor or attempt to induce any customer to terminate its
relationship with the Employer or to not enter into a relationship with the
Employer, as the case may be; or

             (iii) solicit or attempt to solicit the employment of any employee
of the Employer, or any person employed by the Employer during the prior six (6)
month period, or attempt to solicit or induce any such employee or person to
leave the employ of the Employer.

         (c) That in the event any provision of this Section 6 shall be
challenged by Executive or deemed to be unenforceable by a court of competent
jurisdiction, ManTech's obligation to make payments under Section 5(b) shall
immediately cease, and Executive shall reimburse ManTech any payments previously
received pursuant to Section 5(b) hereof.

         7. Survival. Executive agrees that the restrictions in Section 6 shall
            --------
survive the termination of Executive's employment with Employer, notwithstanding
any actual or alleged breach or failure of ManTech to perform its obligations
under this Agreement or otherwise.

         8. Specific Enforcement; Extension of Period.
            -----------------------------------------

         (a) Executive acknowledges that the restrictions contained in Section 6
hereof are reasonable and necessary to protect the legitimate interests of the
Employer and that Employer would not have entered into this Agreement in the
absence of such restrictions. Executive also acknowledges that any breach by him
of Section 6 hereof will cause continuing and irreparable injury to Employer for
which monetary damages would not be an adequate remedy. Executive shall not, in
any action or proceeding by Employer to enforce Section 6 of this Agreement,
assert the claim or defense that an adequate remedy at law exists. In the event
of such breach by Executive, Employer shall have the right to enforce the
provisions of Section 6 of this Agreement by seeking injunctive or other relief
in any court, and this Agreement shall not in any way limit remedies at law or
in equity otherwise available to the Employer.

         (b) THIS AGREEMENT SHALL BE GOVERNED BY AND CONSTRUED IN ACCORDANCE
WITH THE LAWS OF THE COMMONWEALTH OF VIRGINIA, WITHOUT REFERENCE TO THE
PRINCIPLES OF CONFLICTS OF LAW EXCEPT TO THE EXTENT SUCH PRINCIPLES PERMIT THE
APPLICATION OF VIRGINIA LAW OR JURISDICTION AND VENUE IN COURTS WITHIN VIRGINIA.
ANY DISPUTE HEREUNDER SHALL BE LITIGATED IN FEDERAL DISTRICT COURT IN THE
EASTERN DISTRICT OF VIRGINIA OR, IF JURISDICTION CANNOT BE OBTAINED IN SUCH
COURT, IN THE STATE COURT WHOSE JURISDICTION INCLUDES THE PRINCIPAL EXECUTIVE
OFFICE OF MANTECH.

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                (c) Except as otherwise expressly set forth in Section 6(c), all
provisions of this Agreement are intended to be severable. In the event any
provision or restriction contained herein is held to be invalid or unenforceable
in any respect, in whole or in part, such finding will in no way affect the
validity or enforceability of any other provision of this Agreement, except as
otherwise expressly set forth in Section 6(c). The parties hereto further agree
that any such invalid or unenforceable provision will be deemed modified so that
it will be enforced to the greatest extent permissible under law, and to the
extent that any court of competent jurisdiction determines any restriction
herein to be unreasonable in any respect, such court shall limit this Agreement
to render it reasonable in light of the circumstances in which it was entered
into and specifically enforce this Agreement as limited.

                9.  Miscellaneous.

                (a) This Agreement by and between the Executive and ManTech
constitutes the entire agreement between the parties hereto with respect to the
Executive's employment, and supersedes and is in full substitution for any and
all prior understandings or agreements, whether oral or written, with respect to
the Executive's employment.

                (b) ManTech may withhold from any amounts payable to the
Executive hereunder all federal, state, city or other taxes that ManTech may
reasonably determine are required to be withheld pursuant to any applicable law
or regulation.

                (c) This Agreement may be executed by facsimile signature and in
several counterparts, each of which shall be deemed an original, but all of
which shall constitute one and the same instrument.

                (d) The headings in this Agreement are inserted for convenience
of reference only and shall not be a part of or control or affect the meaning of
any provision hereof.

                IN WITNESS WHEREOF, the parties have executed this Agreement
as of the date first written above.

Executive                                   MANTECH INTERNATIONAL CORPORATION

_____________________________________       ___________________________________
signature                                   Print Name: John A. Moore, Jr.
                                            Title: Executive Vice President,
                                                   Chief Financial Officer and
_____________________________________              Treasurer
print name

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                                  Attachment A

                              Non-Cash Compensation

     .    First class business travel, including such expenses for Executive's
          spouse when traveling together on ManTech business;

     .    business travel insurance;

     .    the lease of an executive type of vehicle for business and personal
          use;

     .    the portion of an employee's time spent on non-corporate matters on
          behalf of Executive (including attending to chauffeur/valet services
          and other assistance as required from time to time);

     .    reimbursement of certain cell phone and home telephone/fax services;

     .    various tax, legal and estate planning services relating to the equity
          holdings of ManTech's stock by Executive and Executive's family;

     .    matching contributions to the company's 401(k);

     .    payments for term life insurance;

     .    payment of premiums under one or more split dollar life insurance
          policies;

     .    a minimum contribution of $50,000 per year to Executive's supplemental
          executive retirement plan; and

     .    initiation and periodic dues associated with executive clubs and
          memberships, including, without limitation, the Tower Club, Washington
          Golf and Country Club and the Annapolis Yacht Club.



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