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                                                                   EXHIBIT 10.31

                    EIGHTH LOAN MODIFICATION AGREEMENT

      This Eighth Loan Modification Agreement (this "Loan Modification
Agreement") is entered into as of December 30, 2005, effective as of December
24, 2005, by and between SILICON VALLEY BANK, a California-chartered bank, with
its principal place of business at 3003 Tasman Drive, Santa Clara, California
95054 and with a loan production office located at One Newton Executive Park,
Suite 200, 2221 Washington Street, Newton, Massachusetts 02462 ("Bank") and ART
TECHNOLOGY GROUP, INC., a Delaware corporation with its principal place of
business at 25 First Street, Cambridge, Massachusetts 02141 ("Borrower").

1. DESCRIPTION OF EXISTING INDEBTEDNESS AND OBLIGATIONS. Among other
indebtedness and obligations which may be owing by Borrower to Bank, Borrower is
indebted to Bank pursuant to a loan arrangement dated as of June 13, 2002,
evidenced by, among other documents, a certain Amended and Restated Loan and
Security Agreement dated as of June 13, 2002, between Borrower and Bank, as
amended by a certain First Loan Modification Agreement dated as of September 27,
2002, as further amended by a certain Amendment dated as of October 4, 2002, as
further amended by a certain Second Loan Modification Agreement dated as of
December 24, 2002, as further amended by a certain Third Loan Modification
Agreement dated as of October 20, 2003, as further amended by a certain Fourth
Loan Modification Agreement dated November 26, 2003, as further amended by a
certain Letter Agreement dated June 16, 2004, as further amended by a certain
Fifth Loan Modification Agreement dated June 30, 2004, as further amended by a
certain Sixth Loan Modification Agreement dated November 24, 2004, and as
further amended by a certain Seventh Loan Modification Agreement dated December
21, 2004 (as amended, the "Loan Agreement"). Capitalized terms used but not
otherwise defined herein shall have the same meaning as in the Loan Agreement.

2. DESCRIPTION OF COLLATERAL. Repayment of the Obligations is secured by the
Collateral as described in the Loan Agreement (together with any other
collateral security granted to Bank, the "Security Documents").

Hereinafter, the Security Documents, together with all other documents
evidencing or securing the Obligations shall be referred to as the "Existing
Loan Documents".

3. DESCRIPTION OF CHANGE IN TERMS.

      A.    Modifications to Loan Agreement.

            1.    The Loan Agreement shall be amended by deleting the following
                  definitions appearing in Section 13 thereof, in their
                  entirety:

                        ""REVOLVING MATURITY DATE" is December 24, 2005."

                  and inserting in lieu thereof the following:

                        ""REVOLVING MATURITY DATE" is February 7, 2006."

4. FEES. Borrower shall reimburse Bank for all reasonable legal fees and
expenses incurred in connection with this amendment to the Existing Loan
Documents.

5. RATIFICATION OF INTELLECTUAL PROPERTY SECURITY AGREEMENT. Borrower hereby
ratifies, confirms and reaffirms, all and singular, the terms and conditions of
a certain Intellectual Property Security Agreement dated as of June 13, 2002
between Borrower and Bank, and acknowledges, confirms and agrees that said
Intellectual Property Security Agreement contains an accurate and complete
listing of all Intellectual Property Collateral as defined in said Intellectual
Property Security Agreement (with the exception of the Intellectual Property
Collateral set forth on Schedule 5 attached hereto) as of June 13, 2002 or any
subsequent amendment thereto and shall remain in full force and effect.

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6. RATIFICATION OF PERFECTION CERTIFICATE. Borrower hereby ratifies, confirms
and reaffirms, all and singular, the terms and disclosures contained in a
certain Perfection Certificate dated as of June 13, 2002 between Borrower and
Bank, and acknowledges, confirms and agrees the disclosures and information
Borrower provided to Bank in said Perfection Certificate has not changed, as of
the date hereof.

7. CONSISTENT CHANGES. The Existing Loan Documents are hereby amended wherever
necessary to reflect the changes described above.

8. RATIFICATION OF LOAN DOCUMENTS. Borrower hereby ratifies, confirms, and
reaffirms all terms and conditions of all security or other collateral granted
to the Bank, and confirms that the indebtedness secured thereby includes,
without limitation, the Obligations.

9. NO DEFENSES OF BORROWER. Borrower agrees that, as of this date, it has no
defenses against the obligations to pay any amounts under the Obligations.

10. CONTINUING VALIDITY. Borrower understands and agrees that in modifying the
existing Obligations, Bank is relying upon Borrower's representations,
warranties, and agreements, as set forth in the Existing Loan Documents. Except
as expressly modified pursuant to this Loan Modification Agreement, the terms of
the Existing Loan Documents remain unchanged and in full force and effect.
Bank's agreement to modifications to the existing Obligations pursuant to this
Loan Modification Agreement in no way shall obligate Bank to make any future
modifications to the Obligations. Nothing in this Loan Modification Agreement
shall constitute a satisfaction of the Obligations. It is the intention of Bank
and Borrower to retain as liable parties all makers of Existing Loan Documents,
unless the party is expressly released by Bank in writing. No maker will be
released by virtue of this Loan Modification Agreement.

11. COUNTERSIGNATURE. This Loan Modification Agreement shall become effective
only when it shall have been executed by Borrower and Bank.

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      This Loan Modification Agreement is executed as a sealed instrument under
the laws of the Commonwealth of Massachusetts as of the date first written
above.

BORROWER:                                         BANK:

ART TECHNOLOGY GROUP, INC.                        SILICON VALLEY BANK

By: /s/ Julie M. B. Bradley                       By: /s/ Bradley Holt
    ----------------------------------                --------------------------
Name: Julie M. B. Bradley                         Name: Bradley Holt
Title: Chief Financial Officer                    Title: Relationship Manager