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Exhibit 10.15

Tetra Tech, Inc.

2002 Stock Option Plan

1. Purpose.

    The purpose of the Tetra Tech, Inc. 2002 Stock Option Plan ("Plan") is to
promote the interests of Tetra Tech, Inc. ("Company") and its stockholders by
enabling the Company to offer Participants an opportunity to acquire an equity
interest in the Company so as to better attract, retain, and reward its
employees, directors (excluding nonemployee directors) and other persons
providing services to the Company and, accordingly, to strengthen the mutuality
of interests between Participants and the Company's stockholders by providing
Participants with a proprietary interest in pursuing the Company's long-term
growth and financial success.

2. Definitions.

    For purposes of this Plan, the following terms shall have the meanings set
forth below.

    (a) "Board" means the Board of Directors of Tetra Tech, Inc.

    (b) "Code" means the Internal Revenue Code of 1986, and the applicable
regulations thereunder. Reference to any specific section of the Code shall be
deemed to be a reference to any successor provision.

    (c) "Committee" means the committee appointed by the Board, if any, to
administer this Plan as permitted by Section 5 below or, if no such committee is
appointed, the Board.

    (d) "Common Stock" means the common stock of the Company or any security
issued in substitution, exchange, or in lieu thereof.

    (e) "Company" means Tetra Tech, Inc., a Delaware corporation, or any
successor corporation. Except where the context indicates otherwise, the term
"Company" shall include its Subsidiaries.

    (f)  "Disabled" means permanent and total disability, as defined in Code
Section 22(e)(3).

    (g) "Exchange Act" means the Securities Exchange Act of 1934.

    (h) "Fair Market Value" of Common Stock for any day shall be determined in
accordance with the following rules.

     (i) If the Common Stock is admitted to trading or listed on a national
securities exchange, the last reported sale price on that day regular way, or if
no such reported sale takes place on that day, the average of the last reported
bid and ask prices on that day regular way, in either case on the principal
national securities exchange on which the Common Stock is admitted to trading or
listed.

    (ii) If not listed or admitted to trading on any national securities
exchange, the last sale price regular way on that day reported on the Nasdaq
National Market ("Nasdaq National Market") of the Nasdaq Stock Market ("NSM")
or, if no such reported sale takes place on that day, the average of the closing
bid and ask prices regular way on that day.

    (iii) If not traded or listed on a national securities exchange or included
in the Nasdaq National Market, the last reported sale price on that day regular
way, or if no such reported sale takes place on that day, the average of the
closing bid and ask prices regular way on that day reported by the NSM, or any
comparable system on that day.

    (iv) If the Common Stock is not included in (i), (ii) or (iii) above, the
last reported sale price on that day regular way, or if no such reported sale
takes place on that day, the closing

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bid and ask prices regular way on that day as furnished by any member of the
National Association of Securities Dealers, Inc. ("NASD") selected from time to
time by the Company for that purpose.

If the national securities exchange, Nasdaq National Market, NSM or NASD as
applicable, are closed on such date, the "Fair Market Value" shall be determined
as of the last preceding day on which the Common Stock was traded or for which
bid and ask prices are available. In the case of an Incentive Stock Option,
"Fair Market Value" shall be determined without reference to any restriction
other than one that, by its terms, will never lapse.

    (i)  "Incentive Stock Option" means an option to purchase Common Stock that
is an incentive stock option within the meaning of Code Section 422.

    (j)  "Insider" means a person who is subject to Section 16 of the Exchange
Act.

    (k) "Non-Qualified Stock Option" means any option to purchase Common Stock
that is not an Incentive Stock Option.

    (l)  "Option" means an Incentive Stock Option or a Non-Qualified Stock
Option.

    (m) "Participant" means a person who was been granted an Option under the
Plan.

    (n) "Plan" means this Tetra Tech, Inc. 2002 Stock Option Plan, as it may be
amended from time to time.

    (o) "Severance" means, with respect to a Participant, the termination of the
Participant's provision of services to the Company as an employee or independent
contractor, whether by reason of death, disability, or any other reason. For
purposes of determining the exercisability of an Incentive Stock Option, a
Participant who is on a leave of absence that exceeds ninety (90) days will be
considered to have incurred a Severance on the ninety-first (91st) day of the
leave of absence, unless the Participant's rights to reemployment are guaranteed
by statute or contract. However, a Participant will not be considered to have
incurred a Severance because of a transfer of employment between the Company and
a Subsidiary (or vice versa).

    (p) "Subsidiary" means any corporation or entity in which Tetra Tech, Inc.,
directly or indirectly, controls fifty percent (50%) or more of the total voting
power of all classes of its stock having voting power, as determined in
accordance with the rules of Code Section 424(f).

    (q) "Ten Percent Shareholder" means any person who owns (after taking into
account the constructive ownership rules of Code Section 424(d)) more than ten
percent (10%) of the stock of the Tetra Tech, Inc. or of any of its
Subsidiaries.

3. Eligibility.

    All employees (including employee directors) and other persons providing
bona fide services to the Company or any Subsidiary are eligible to receive
Options under this Plan. However, Incentive Stock Options may only be granted to
employees of the Company or of a Subsidiary.

4. Substitute Options.

    In the event the Company acquires another entity, the Committee may
authorize the issuance of Options ("Substitute Options") to employees and other
persons in substitution of stock options previously granted to them in
connection with their performance of services for such acquired entity upon such
terms and conditions as the Committee shall determine but which shall not be
contrary to applicable law, taking into account the limitations of Code
Section 424(a) in the case of any Substitute Option that is intended to be an
Incentive Stock Option.

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5. Administration.

    (a) This Plan shall be administered by a Committee consisting of two or more
members of the Board appointed by the Board. The Board may remove members from,
or add members to, the Committee at any time. To the extent possible and
advisable, the Committee shall be composed of individuals who satisfy Rule 16b-3
under the Exchange Act and Code Section 162(m). Notwithstanding anything herein
to the contrary, any action which may be taken by the Committee may also be
taken by the Board.

    (b) The Committee may conduct its meetings in person or by telephone. A
majority of the members of the Committee shall constitute a quorum, and any
action shall constitute the action of the Committee if it is authorized by:

     (i) A majority of the members present at any meeting conducted in
accordance with the Company's bylaws; or

    (ii) The unanimous consent of all of the members in writing without a
meeting.

    (c) The Committee is authorized to interpret this Plan and to adopt rules
and procedures relating to the administration of this Plan. All actions of the
Committee in connection with the interpretation and administration of this Plan
shall be binding upon all parties.

    (d) Subject to the limitations set forth below, the Committee is expressly
authorized to make such modifications to this Plan and the Options granted
hereunder as are necessary to effectuate the intent of this Plan as a result of
any changes in the tax, accounting, or securities laws treatment of
Participants, the Company and the Plan.

    (e) The Committee may delegate its responsibilities to others under such
conditions and limitations as it may prescribe, except that the Committee may
not delegate its authority with regard to the granting of Options to Insiders if
that would cause such grants to fail to satisfy Rule 16b-3 under the Exchange
Act or Code Section 162(m).

6. Effective Date.

    This Plan shall be effective on December 19, 2001, provided it is approved
by the holders of a majority of the Common Stock, at the Company's 2002 Annual
Meeting. If the Plan is not approved by the stockholders at that meeting, the
Plan and all Options issued under the Plan will terminate. The approval by the
stockholders must relate to:

    (a) The class of individuals who are entitled to receive Incentive Stock
Options; and

    (b) The maximum number of shares of Common Stock that may be issued under
the Plan, except as adjusted pursuant to Section 13 of this Plan.

    If either of those items is changed, the approval of the stockholders must
again be obtained.

7. Termination of Plan.

    This Plan shall terminate on December 18, 2011, except with respect to
Options then outstanding. However, the Board may elect to terminate the Plan on
a prior date. The termination of this Plan shall not adversely affect the rights
of any Participant with respect to any Option outstanding as of the time of such
termination.

8. Shares Subject to this Plan.

    (a) The maximum number of shares of Common Stock which may be issued
pursuant to this Plan shall be Four Million (4,000,000). Upon the expiration or
termination of an outstanding

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Option which shall not have been exercised in full, the shares of Common Stock
remaining unissued under the Option shall again become available for use under
the Plan.

    (b) The maximum number of shares of Common Stock that may be issuable
pursuant to Options granted during any calendar year to any Participant is two
hundred thousand (200,000) shares. For purposes of determining the maximum
number of shares that may be issued to a single Participant, shares subject to a
terminated Option shall be considered outstanding.

    (c) In the event a Participant pays part or all of the exercise price of an
Option by surrendering shares of Common Stock that the Participant previously
acquired, only the number of shares issuable to the Participant in excess of
those surrendered shall be taken into account for purposes of determining the
maximum number of shares that may be issued under the Plan, both as to that
Participant and in the aggregate (to all Participants).

9. Form of Options.

    (a) Options shall be granted under this Plan on such terms and in such form
as the Committee may approve, which shall not be inconsistent with the
provisions of this Plan, but which need not be the same for each such grant.
Options may be either Nonqualified Stock Options or Incentive Stock Options.

    (b) The terms and conditions of each Option shall include, in addition to
such other terms and conditions as may be established by the Committee, (i) the
per share exercise price of such Option in accordance with subparagraph
(c) below, (ii) the termination date of such Option, and (iii) the effect on
such Option of the Participant's Severance. Subject to all other provisions of
this Plan, each Option shall become exercisable (i) as to one-fourth (1/4) of
the full number of shares subject thereto one year after the date of grant and
(ii) as to the balance in thirty-six (36) equal cumulative monthly installments
following such first anniversary date, or in such other installments and at such
other intervals as the Board or the Committee may in any specific case otherwise
determine in granting such Option. Any Option shall be exercisable following the
date of the Participant's Severance only to the extent (if at all) such Option
was exercisable on the date of Severance.

    (c) The exercise price per share of Common Stock purchasable under an Option
shall be set forth in the Option, which in all cases shall be at least equal to
the Fair Market Value of the Common Stock on the date of the grant. The exercise
price of an Incentive Stock Option granted to a Ten Percent Shareholder shall be
no less than one hundred ten percent (110%) of the Fair Market Value of the
Common Stock on the date of the grant.

    (d) Any Option which is intended, as evidenced by its designation, as an
Incentive Stock Option shall be made subject to such terms and conditions as may
be required for such Option to qualify as an Incentive Stock Option. Incentive
Stock Options granted under this Plan shall also include a requirement that the
Participant receiving such Incentive Stock Option must notify the Company if he
or she disposes of Common Stock acquired pursuant to the exercise thereof prior
to the expiration of the holding periods prescribed in Section 422(a)(1) of the
Code.

    (e) If an Option is intended to be exempt from the million dollar
($1,000,000) compensation deduction limitation of Code Section 162(m), in
addition to the exercise price requirement of subparagraph (c), the grant must
be made by a committee composed exclusively of two (2) or more "outside
directors" as that term is defined under Code Section 162(m).

10. Modification of Options.

    (a) The Committee may modify any existing Option as it deems appropriate.
Such authority shall include, without limitation, the right to accelerate the
right to exercise any Option, extend or

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renew any Option, and modify any restrictions with respect to any Option.
However, in no event will the exercise price of any outstanding Option be
reduced or repriced (as determined under applicable accounting standards),
including any repricing effected by issuing replacement stock options for
outstanding stock options that have an exercise price greater than the Fair
Market Value of the Common Stock, without first obtaining stockholder approval.

    (b) No modification may be made to any Option that would adversely affect
the rights of the Participant holding the Option without the Participant's
consent. Further, no such modification may be made within taking into
consideration any consequences under Code Section 162(m).

    (c) In the event the Committee amends the terms of an Option so that it no
longer qualifies as an Incentive Stock Option, the limitations imposed upon the
Option under the Code and the Plan by virtue of it (formerly) qualifying as an
Incentive Stock Option shall no longer apply, to the extent specified in the
amendment.

    (d) Whether a modification of an existing Incentive Stock Option will be
treated as the issuance of a new Incentive Stock Option will be determined in
accordance with the rules of Code Section 424(h). Whether a modification of an
existing Option granted to an Insider will be treated as a new grant for
purposes of Section 16 of the Exchange Act will be determined in accordance with
Rule 16b-3 under the Exchange Act.

11. Termination of Options.

    Except to the extent the terms of an Option require its prior termination,
each Option shall terminate on the earliest of the following dates:

    (a) The date which is ten (10) years from the date on which the Option is
granted or five (5) years in the case of an Incentive Stock Option granted to a
Ten Percent Shareholder.

    (b) The date which is one (1) year from the date of the Severance of the
Participant to whom the Option was granted, if the Participant was Disabled at
the time of Severance.

    (c) The date which is one (1) year from the date of the Severance of the
Participant to whom the Option was granted, if the Participant's death occurs:

     (i) While the Participant is employed by the Company; or

    (ii) Within three (3) months following the Participant's Severance.

    (d) In the case of any Severance other than one described in subparagraphs
(b) or (c) above, the date that is three (3) months from the date of the
Participant's Severance.

12. Non-transferability of Options.

    No Option under this Plan shall be assignable or transferable except by will
or the laws of descent and distribution.

13. Adjustments

    (a) In the event of any change in the capitalization of the Company
affecting its Common Stock (e.g., a stock split, reverse stock split, stock
dividend, recapitalization, combination, or reclassification), the Committee
shall authorize such adjustments as it may deem appropriate with respect to:

     (i) The maximum number of shares of Common Stock that may be issued under
this Plan;

    (ii) The number of shares of Common Stock covered by each outstanding
Option;

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    (iii) The exercise price per share in respect of each outstanding Option;
and

    (iv) The maximum number of shares that may be issued to a single individual.

    (b) The Committee may also make such adjustments in the event of a spin-off
or other distribution of Company assets to stockholders, other than normal cash
dividends.

14. Change in Control.

    In connection with any merger or consolidation of the Company with or into
another entity in which the Company is not the surviving corporation or as a
result of which the Common Stock ceases or will cease to be publicly traded, the
Committee may, but shall not be required to, authorize the termination of all
outstanding Options upon the consummation of such merger or consolidation.
However, as a condition to such termination, all restrictions on the
exercisability of such Options (i.e., vesting provisions) shall be eliminated
and the holders thereof shall be given at least twenty (20) days prior to such
termination to exercise their Options without regard to any such restrictions.

15. Amendment and Termination.

    (a) The Board may at any time amend or terminate this Plan. However, no
modification may be made to the Plan that would impair the rights of the
Participant holding an Option without the Participant's consent.

    (b) Without the approval of the holders of a majority of the Common Stock,
the Board may not amend the provisions of this Plan for the purpose of:

     (i) Modifying the class of individuals entitled to receive Options;

    (ii) Increasing the maximum number of shares of Common Stock that may be
issued under the Plan, except as provided in Section 13 of this Plan; or

    (iii) Materially increasing the benefits which accrue to Participants under
this Plan.

16. Withholding.

    (a) The Company shall have the right to take such actions as may be
necessary to satisfy its tax withholding obligations relating to the operation
of this Plan.

    (b) If Common Stock that was surrendered by the Participant is used to
satisfy the Company's tax withholding obligations, the stock shall be valued
based on its Fair Market Value when the tax withholding is required to be made.
The maximum number of shares that may be withheld is the minimum number of
shares necessary to satisfy the applicable tax withholding rules.

17. Additional Rights.

    (a) Neither the adoption of this Plan nor the granting (or exercise) of any
Option shall:

     (i) Affect or restrict in any way the power of the Company to undertake any
corporate action otherwise permitted under applicable law;

    (ii) Confer upon any Participant the right to continue performing services
for the Company; or

    (iii) Interfere in any way with the right of the Company to terminate the
services of any Participant at any time, with or without cause, subject to such
other contractual obligations as may exist.

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    (b) No Participant shall have any rights as a stockholder with respect to
any shares covered by an Option granted to the Participant until the date a
certificate for such shares has been issued to the Participant following the
exercise of the Option.

18. Securities Law Restrictions.

    (a) No shares of Common Stock shall be issued under this Plan unless the
Committee shall be satisfied that the issuance will be in compliance with
applicable federal and state securities laws, as well as the requirements of any
stock exchange or quotation system upon which the Common Stock is listed or
quoted.

    (b) The Committee may require certain investment (or other) representations
and undertakings by the Participant (or other person exercising an Option by
reason of the death of the Participant) in order to comply with applicable law.

    (c) Certificates for shares of Common Stock delivered under this Plan may be
subject to such restrictions as the Committee may deem advisable. The Committee
may cause a legend to be placed on the certificates to refer to these
restrictions.

19. Indemnification.

    (a) To the maximum extent permitted by law, the Company shall indemnify each
member of the Board, as well as any other employee of the Company with duties
under this Plan, against expenses (including any amount paid in settlement)
reasonably incurred by the individual in connection with any claims against him
or her by reason of the performance of the individual's duties under this Plan,
unless the losses are due to the individual's gross negligence or lack of good
faith.

    (b) The Company will have the right to select counsel and to control the
prosecution or defense of the suit.

    (c) In the event that more than one person who is entitled to
indemnification is subject to the same claim, all such persons shall be
represented by a single counsel, unless such counsel advises the Company in
writing that he or she cannot represent all such persons under applicable rules
of professional responsibility.

    (d) The Company will not be required to indemnify any person for any amount
incurred through any settlement unless the Company consents in writing to the
settlement.

20. Governing Law. This Plan and all actions taken thereunder shall be governed
by and construed in accordance with the laws of the State of Delaware.

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QuickLinks

Exhibit 10.15
Tetra Tech, Inc. 2002 Stock Option Plan