SIXTH MODIFICATION TO AMENDED AND RESTATED BUSINESS LOAN AND SECURITY AGREEMENT AND OTHER LOAN DOCUMENTS

Exhibit 10.1

SIXTH MODIFICATION TO AMENDED AND RESTATED BUSINESS LOAN AND SECURITY

AGREEMENT AND OTHER LOAN DOCUMENTS

THIS SIXTH MODIFICATION TO AMENDED AND RESTATED BUSINESS LOAN AND SECURITY AGREEMENT AND OTHER LOAN DOCUMENTS (this “Modification”), dated as of February     , 2008, is made by and among (i) CITIZENS BANK OF PENNSYLVANIA, a Pennsylvania state chartered bank (“Citizens Bank”), acting in its capacity as the agent for the Lenders (the “Agent”), having offices at 8521 Leesburg Pike, Suite 405, Vienna, Virginia 22182; (ii) CITIZENS BANK, acting in its capacity as a Lender, and each other “Lender” party to the hereinafter defined Loan Agreement (each, a “Lender” and collectively, the “Lenders”); and (iii) ICF CONSULTING GROUP, INC., a Delaware corporation (the “Primary Operating Company”), ICF INTERNATIONAL, INC., a Delaware corporation (the “Parent Company”), and each other “Borrower” party to the Loan Agreement (together with the Primary Operating Company and the Parent Company, each, a “Borrower” and collectively, the “Borrowers”), each having offices at 9300 Lee Highway, Fairfax, Virginia 22031. Capitalized terms used but not defined herein shall have the meanings attributed to such terms in the Loan Agreement.

W I T N E S S E T H    T H A T:

WHEREAS, pursuant to the terms of a certain Amended and Restated Business Loan and Security Agreement dated as of October 5, 2005 (as amended, modified or restated from time to time, the “Loan Agreement”), by and among the Borrowers, the Agent and the Lenders, the Borrowers originally obtained loans and certain other financial accommodations (collectively, the “Loan”) from the Lenders in the aggregate maximum principal amount of Seventy-five Million and No/100 Dollars ($75,000,000.00) comprised of (a) Facility A in the maximum principal amount of Forty-five Million and No/100 Dollars ($45,000,000.00), (b) Facility B in the original principal amount of Twenty-two Million and No/100 Dollars ($22,000,000.00), and (c) Facility C in the original principal amount of Eight Million and No/100 Dollars ($8,000,000.00); and

WHEREAS, the Loan is evidenced by the Notes and secured by, among other things, the collateral described in the Loan Agreement; and

WHEREAS, pursuant to the terms of a certain First Modification to Amended and Restated Business Loan and Security Agreement and Other Loan Documents dated as of March 14, 2006, the Lenders agreed to a temporary allowance of up to Six Million and No/100 Dollars ($6,000,000.00) for over-advances for the benefit of the Borrowers; and

WHEREAS, pursuant to the terms of a certain Second Modification to Amended and Restated Business Loan and Security Agreement and Other Loan Documents dated as of August 25, 2006, the maximum principal amount of Facility A was increased from Forty-five Million and No/100 Dollars ($45,000,000.00) to Sixty-five Million and No/100 Dollars ($65,000,000.00), and the Parent Company consummated an initial public offering of its common stock, the proceeds of which were used, in part, to repay all amounts then outstanding and unpaid under Facility A, Facility B, Facility C and the Swing Line Facility; and

WHEREAS, pursuant to the terms of a certain Third Modification to Amended and Restated Business Loan and Security Agreement and Other Loan Documents dated as of December 29, 2006, the Agent and the Lenders agreed to modify certain provisions of the Loan Agreement, including without limitation, provisions pertaining to pricing, interest rate protection arrangements and other provisions more particularly described therein; and

WHEREAS, pursuant to the terms of a certain Fourth Modification to Amended and Restated Business Loan and Security Agreement and Other Loan Documents dated as of June 28, 2007, Z-Tech Corporation was joined as a “Borrower” party to the Loan Agreement and the other Loan Documents, the maximum principal amount of Facility A was increased from Sixty-five Million and No/100 Dollars ($65,000,000.00) to Ninety-five Million and No/100 Dollars ($95,000,000.00) and the maximum principal amount of the Swing Line Facility was increased from Ten Million and No/100 Dollars ($10,000,000.00) to Twenty Million and No/100 Dollars ($20,000,000.00); and

WHEREAS, pursuant to the terms of a certain Fifth Modification to Amended and Restated Business Loan and Security Agreement and Other Loan Documents dated as of December 3, 2007, among other things, Simat, Helliesen & Eichner, Inc., and its subsidiaries were joined as “Borrower” parties to the Loan Agreement and the other Loan Documents, the borrowing base and other provisions of the Loan Agreement were amended and the maximum principal amount of Facility A was increased from Ninety-five Million and No/100 Dollars ($95,000,000.00) to One Hundred Fifteen Million and No/100 Dollars ($115,000,000.00); and

WHEREAS, the Borrowers have requested that the Agent and the Lenders (a) consent to the Borrowers’ proposed acquisition (the “Jones Acquisition”) of Jones & Stokes Associates, Inc., a California corporation (“Jones”), pursuant to that certain Agreement and Plan of Merger dated as of January 23, 2008 (the “Jones Acquisition Agreement”), by and among the Parent Company, the Primary Operating Company, ICF Consulting Group Acquisition, Inc., Jones, the shareholders of Jones and John W. Cowdery as the shareholder representative, (b) further increase the maximum principal amount of Facility A from One Hundred Fifteen Million and No/100 Dollars ($115,000,000.00) to One Hundred Twenty-five Million and No/100 Dollars ($125,000,000.00), the proceeds of which will be used, in part, to finance the Jones Acquisition, as well as the transactional costs and expenses related thereto, and (c) amend certain other terms and provisions set forth in and/or contemplated by the Loan Agreement; and

WHEREAS, the Agent and the Lenders have agreed to grant the Borrowers’ request, subject to the terms and conditions set forth herein; and

WHEREAS, the Borrowers, the Agent and the Lenders desire to enter into this Modification to memorialize the agreements and understanding of the parties with respect to the foregoing matters, as hereinafter provided.

NOW THEREFORE, for Ten Dollars ($10.00) and other good and valuable consideration, the receipt and sufficiency of which are hereby acknowledged, the parties hereto agree as follows:

1. Recitals. The foregoing recitals are hereby incorporated herein by this reference and made a part hereof, with the same force and effect as if fully set forth herein.

2. Loan Increase; Promissory Notes.

(A) Subject to the terms and provisions set forth in this Modification, the Facility A Commitment Amount is hereby increased from One Hundred Fifteen Million and No/100 Dollars ($115,000,000.00) to One Hundred Twenty-five Million and No/100 Dollars ($125,000,000.00).

(B) Simultaneously with the execution and delivery of this Modification, the Borrowers shall execute and deliver to the Agent, in form and substance reasonably satisfactory to the Agent and its counsel: (a) one or more note modification agreements and/or substitute promissory notes with respect to Facility A, which shall evidence the increase to the Facility A Commitment Amount, as described in this Modification; (b) an opinion of in-house counsel relating to the consummation of the Jones Acquisition; (c) certified resolutions and consents, authorizing the increase to the Commitment Amount and related matters; (d) UCC, judgment, pending litigation, bankruptcy and tax lien search results for Jones; and (e) such other documents, instruments and agreements as the

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Agent and/or the Lenders may reasonably request; it being understood and agreed that in the event that the parties do not consummate the contemplated amendment and restatement of the Loan Agreement on or prior to March 31, 2008, the Agent may request, in its sole discretion, one or more opinions, certificates, UCC, judgment, pending litigation, bankruptcy and tax lien searches and other similar information relating to the Borrowers and this Modification.

(C) Each of the parties hereto acknowledges and agrees that: (i) any and all collateral securing the Obligations in whole or in part shall secure the Obligations, as increased, expanded and extended pursuant to this Modification, and all Loan Documents are hereby deemed amended accordingly; and (ii) the additional Loan proceeds of Facility A made available pursuant to this Modification shall be advanced from time to time in accordance with and subject to the applicable provisions of the Loan Agreement.

3. Definitional Amendments.

The definition of “Facility A Commitment Amount” set forth in the section of the Loan Agreement titled “Certain Definitions” is hereby deleted in its entirety and replaced with the following:

““Facility A Commitment Amount” shall mean One Hundred Twenty-five Million and No/100 Dollars ($125,000,000.00), or if such amount shall be reduced pursuant to this Agreement, such lesser amount.”

4. Acquisition Consent. The Agent and the Lenders hereby (a) consent to the acquisition by the Primary Operating Company of all of the issued and outstanding capital stock of Jones, and (b) acknowledge that such acquisition shall not count against the dollar basket with respect to any Permitted Acquisition set forth in Section 7.1(d)(ii)(H) of the Loan Agreement, subject to the terms, covenants, agreements and conditions set forth in this Modification, including without limitation, the following:

(i) The Primary Operating Company shall have acquired all of the issued and outstanding capital stock of Jones, free and clear of all liens, claims, encumbrances and any other restrictions or limitations on transfer thereof (other than Permitted Liens), and the Jones Acquisition shall have been consummated in accordance with the Jones Acquisition Agreement, subject to the grant of any waivers thereunder or modifications thereto (a copy of which shall be provided to the Agent and its counsel prior to the Borrowers’ use of any Loan proceeds for the Jones Acquisition);

(ii) Simultaneously with the Jones Acquisition, Jones shall have become joined to the Loan Agreement, the Notes and the other Loan Documents as a “Borrower” or “Maker” thereunder (as applicable) by executing this Modification and all other documents, instruments and agreements requested by the Agent and the Lenders in connection therewith;

(iii) the Primary Operating Company shall have delivered to the Agent a Borrowing Base/Non-Default Certificate evidencing a minimum availability under Facility A of at least Ten Million and No/100 Dollars ($10,000,000.00) as of the date of funding of the Jones Acquisition;

(iv) The Borrowers shall have delivered to the Agent and its counsel, in form and substance satisfactory to the Agent and its counsel in all respects, each of the following items:

(A) a true, correct and complete copy of the fully executed Jones Acquisition Agreement, together with all schedules and exhibits attached thereto and/or referenced therein and all other documents, instruments and agreements executed, issued and/or delivered in connection with the Jones Acquisition;

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(B) the articles of incorporation, certificate of formation (or comparable formation documents) of Jones, together with all amendments thereto, recently certified by the applicable governmental authority of the jurisdiction of organization or incorporation;

(C) the by-laws or operating agreements of Jones, together with all amendments thereto, recently certified by a duly authorized corporate officer of Jones;

(D) corporate resolutions of the board of directors (or similar governing body) of Jones, authorizing the execution and delivery of this Modification and related agreements, and the performance of the transactions contemplated hereby, together with an incumbency certificate, certified by a duly authorized corporate officer of Jones;

(E) a recent good standing certificate issued by the jurisdiction of formation or incorporation of Jones, together with recent foreign qualification certificates issued by the comparable state or country office where the nature of Jones’ business requires Jones to be qualified to do business in such state or country;

(F) recent UCC, judgment, pending litigation, bankruptcy and tax lien search results of Jones for each jurisdiction (county and state) where any assets of Jones having a book value in excess of One Hundred Thousand and No/100 Dollars ($100,000.00) are located, and where Jones is organized;

(G) a duly executed and delivered joinder to contribution agreement from Jones;

(H) a pro forma quarterly covenant compliance/non-default certificate in the form attached as Exhibit 5 to the Loan Agreement, reporting results for the quarter ending September 30, 2007;

(I) duly executed and delivered documentation relating to Jones’ execution and delivery of this Modification, the performance by Jones of all transactions contemplated hereby (including, without limitation, the joinder of Jones), the consummation of the Jones Acquisition and such other matters as the Agent or its counsel may require;

(J) one or more opinions of counsel with respect to the consummation of the Jones Acquisition; and

(K) evidence of insurance and related certificates, including but not limited to, fire, hazard, extended coverage, product and other liability, workmen’s compensation, business interruption, umbrella and key man insurance, in form and substance satisfactory to the Agent and its counsel in all respects.

(v) The Agent, for itself and for the ratable benefit of the Lenders, shall have been granted a valid, binding and enforceable first priority perfected lien and security interest (subject only to Permitted Liens) in and to all of Jones’ assets; and

(vi) Not later than the first anniversary of the date hereof, the Borrowers shall cause all primary cash collection accounts (each, a “Primary Cash Collection Account”) of Jones to be maintained with the Agent, and all other primary bank accounts (each, a “Primary Bank Account” and together with the Primary Cash Collection Accounts, each a “Covered Account” and collectively, the “Covered Accounts”) of Jones to be maintained with a Lender. Within ninety (90) days of the date hereof, the Borrowers shall cause any third party depository institution maintaining a Primary Cash Collection Account of Jones, to enter into a wire transfer arrangement with respect to such Primary Cash Collection Account, in form and substance reasonably satisfactory to the Agent; provided, however, that (a) in all events, all Covered Accounts shall be maintained solely with the Agent or a Lender (as applicable) not later than the first anniversary of the date hereof, and (b) this Section shall not require Jones to transfer any Covered Account that would not otherwise be covered by Section 6.8 of the Loan Agreement; and provided, further, that this Section shall not be construed to require Jones to take or omit to take any action or transfer any Covered Account (individually and collectively, the “Excluded Bank Accounts”) that would violate any applicable laws or regulations (including, without limitation, ERISA). It is expressly

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understood and agreed that for so long as Jones shall maintain any Covered Account (other than the Excluded Bank Accounts) with any depository institution other than the Agent or a Lender, then such Covered Account: (a) shall be used solely for the deposit/receipt of cash, checks and other remittances owing to Jones from time to time; (b) shall be at all times, free and clear of any and all liens, claims and encumbrances (other than the security interest of the Agent granted hereby); and (c) shall secure the Obligations.

5. Joinder. Jones is hereby joined as a party to and agrees to be bound by the terms and conditions of the Loan Agreement, the Notes and the other Loan Documents, to the same extent as if it were an original signatory thereto and originally named therein as a Borrower or Maker (as the case may be). Jones hereby makes all of the representations and warranties set forth in the Loan Agreement (as modified or supplemented hereby) and each other Loan Document to which more than one (1) Borrower is a party thereto and grants to the Agent, for the ratable benefit of the Lenders, a valid and enforceable first priority security interest in and to all of its assets constituting Collateral, free and clear of all liens, claims and encumbrances (other than any Permitted Liens). Jones further acknowledges and agrees that it shall be jointly and severally liable for the performance of any and all past, present and future obligations of the Borrower(s) in connection with any of the Note(s), the Loan Agreement and/or the other Loan Documents; it being understood and agreed that any and all references in the Note(s), the Loan Agreement and/or the other Loan Documents to “the Borrower” shall mean Jones, individually and/or collectively with all other Borrowers.

6. Non-Borrower Subsidiaries. Jones is a member of seven (7) joint venture entities as follows (collectively, the “Non-Borrower Subsidiaries”): (i) Bayview Transportation Consultants, LLC, (ii) HDR/Jester Seattle Joint Venture, (iii) Irvington Partners JV, LLC, (iv) JESTER EDAW, LLC, (v) JSR Venture, LLC, (vi) SPK Venture, LLC and (vii) Mooney-Hayes, LLC. Each Borrower hereby acknowledges, covenants, warrants and agrees that it will not transfer any assets to (including by way of capital contribution), make loans to, and no proceeds of the Loan may be used by the Non-Borrower Subsidiaries.

7. Exhibit Substitutions. Schedule 1 and Exhibit 4 attached to the Loan Agreement are hereby deleted in their entirety and Schedule 1 and Exhibit 4 attached hereto substituted in lieu thereof.

8. Updated Schedules. The Borrowers shall have delivered to the Agent and its counsel, in form and substance satisfactory to the Agent and its counsel in all respects, updated schedules to the Loan Agreement, all of which shall be attached and be deemed a part of the Loan Agreement.

9. Expenses. The Borrowers shall have paid to the Agent (for the ratable benefit of the Lenders based on each Lender’s Percentage prior to giving effect to the increase to the Facility A Commitment Amount set forth herein) in immediately available funds, an upfront fee in the amount of Fifteen Thousand and No/100 Dollars ($15,000.00), which, each of the Borrowers acknowledges, has been fully earned as of the date hereof. The Borrowers shall also pay all of the Agent’s costs and expenses associated with this Modification and the transactions referenced herein or contemplated hereby, including, without limitation, the Agent’s reasonable legal fees and expenses.

10. Conditions Precedent. As a condition precedent to the effectiveness of this Modification, the Agent and its counsel shall have received the following, each in form and substance satisfactory to the Agent and its counsel in all respects: (a) a fully executed copy of this Modification; and (b) such other documents, instruments, certificates of good standing, corporate resolutions, limited liability company consents, UCC financing statements, opinions, certifications, schedules to be attached to the Loan Agreement and agreements as the Agent may reasonably request, each in such form and content and from such parties as the Agent shall require.

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11. Miscellaneous.

(i) Each Borrower hereby represents, warrants, acknowledges and agrees that as of the date hereof (i) there are no set-offs, defenses, deductions or counterclaims against and no defaults under any of the Notes, the Loan Agreement or any other Loan Document; (ii) no act, event or condition has occurred which, with notice or the passage of time, or both, would constitute a default under any of the Notes, the Loan Agreement or any other Loan Document; (iii) all of the representations and warranties of the Borrowers contained in the Loan Agreement are true and correct as of the date hereof (except to the extent that such representations and warranties expressly relate solely to an earlier date), unless the Borrowers are unable to remake and redate any such representation or warranty, in which case the Borrowers have previously disclosed the same to the Agent and the Lenders in writing, and such inability does not constitute or give rise to an Event of Default; (iv) all schedules attached to the Loan Agreement with respect to any particular representation and warranty of the Borrowers set forth in the Loan Agreement (as modified) remain true, accurate and complete; (v) all accrued and unpaid interest and fees payable with respect to the Loan have been paid; and (vi) there has been no material adverse change in the business, property or condition (financial or otherwise) of the Borrowers since the date of the most recent financial statements listed on Schedule 5.3.

(ii) The Borrowers, and their respective representatives, successors and assigns, hereby jointly and severally, knowingly and voluntarily RELEASE, DISCHARGE, and FOREVER WAIVE and RELINQUISH any and all claims, demands, obligations, liabilities, defenses, affirmative defenses, setoffs, counterclaims, actions, and causes of action of whatsoever kind or nature, whether known or unknown, which they have, may have, or might have or may assert now or in the future against the Agent and/or the Lenders directly or indirectly, arising out of, based upon, or in any manner connected with any transaction, event, circumstance, action, failure to act, or occurrence of any sort or type, in each case related to, arising from or in connection with the Loan, whether known or unknown, and which occurred, existed, was taken, permitted, or begun prior to the date hereof (including, without limitation, any claim, demand, obligation, liability, defense, counterclaim, action or cause of action relating to or arising from the grant by the Borrowers to the Agent and/or the Lenders of a security interest in or encumbrance on collateral that is, was or may be subject to, or an agreement by which the Borrowers are bound and which contains, a prohibition on further mortgaging or encumbering the same). The Borrowers hereby acknowledge and agree that the execution of this Modification by the Agent and the Lenders shall not constitute an acknowledgment of or an admission by the Agent and/or the Lenders of the existence of any such claims or of liability for any matter or precedent upon which any liability may be asserted.

(iii) Except as expressly set forth herein, nothing contained in this Modification is intended to or shall otherwise act to nullify, discharge, or release any obligation incurred in connection with the Notes, the Loan Agreement and/or the other Loan Documents or to waive or release any collateral given by any Borrower to secure the Notes, nor shall this Modification be deemed or considered to operate as a novation of the Notes, the Loan Agreement or the other Loan Documents. Except to the extent of any express conflict with this Modification or except as otherwise expressly contemplated by this Modification, all of the terms and conditions of the Notes, the Loan Agreement and the other Loan Documents shall remain in full force and effect, and the same are hereby expressly approved, ratified and confirmed. In the event of any express conflict between the terms and conditions of the Notes, the Loan Agreement or the other Loan Documents and this Modification, this Modification shall be controlling and the terms and conditions of such other documents shall be deemed to be amended to conform with this Modification.

(iv) If any term, condition, or any part thereof, of this Modification, the Loan Agreement or of the other Loan Documents shall for any reason be found or held to be invalid or unenforceable by any court or governmental agency of competent jurisdiction, such invalidity or unenforceability shall not affect the remainder of such term, provision or condition nor any other term, provision, or

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condition of this Modification, the Loan Agreement and the other Loan Documents, and this Modification, the Loan Agreement and the other Loan Documents shall survive and be construed as if such invalid or unenforceable term, provision or condition had not been contained therein.

(v) Each Borrower acknowledges that, at all times prior to and through the date hereof, the Agent and the Lenders have acted in good faith and have conducted themselves in a commercially reasonable manner in their relationship with such Borrower in connection with this Modification and in connection with the obligations of the Borrowers to the Agent and the Lenders under the Loan; the Borrowers hereby waiving and releasing any claims to the contrary.

(vi) Each Borrower, Lender and the Agent hereby acknowledges and agrees that, from and after the date hereof, all references to the “Loan Agreement” set forth in any Loan Document shall mean the Loan Agreement, as modified pursuant to this Modification and any other modification of the Loan Agreement dated prior to the date hereof.

(vii) Each Borrower hereby represents and warrants that, as of the date hereof, such Borrower is indebted to the Lenders in respect of the amounts due and owing under the Notes, all such amounts remain outstanding and unpaid and all such amounts are payable in full, without offset, defenses, deduction or counterclaim of any kind or character whatsoever.

(viii) Each Borrower acknowledges (a) that it has participated in the negotiation of this Modification, and no provision of this Modification shall be construed against or interpreted to the disadvantage of any party hereto by any court or other governmental or judicial authority by reason of such party having or being deemed to have structured, dictated or drafted such provision; (b) that it has had access to an attorney of its choosing in the negotiation of the terms of and in the preparation and execution of this Modification, and it has had the opportunity to review, analyze, and discuss with its counsel this Modification, and the underlying factual matters relevant to this Modification, for a sufficient period of time prior to the execution and delivery hereof; (c) that all of the terms of this Modification were negotiated at arm’s length; (d) that this Modification was prepared and executed without fraud, duress, undue influence, or coercion of any kind exerted by any of the parties upon the others; and (e) that the execution and delivery of this Modification is the free and voluntary act of such Borrower.

(ix) This Modification shall be governed by the laws of the Commonwealth of Virginia (without regard to conflict of laws provisions) and shall be binding upon and inure to the benefit of the parties hereto and their respective successors and assigns.

(x) This Modification may be executed in any number of counterparts, each of which shall be deemed an original and all of which together shall be deemed one and the same instrument. Signature pages may be exchanged by facsimile or electronic mail and each party hereto agrees to be bound by its facsimile signature and/or pdf signature.

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IN WITNESS WHEREOF, the undersigned have executed this Modification as of the date first above written.

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