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                                                                    EXHIBIT 31.1

                                  CERTIFICATION

I, Thomas E. Noonan, certify that:

      1.    I have reviewed this quarterly report on Form 10-Q of Internet
            Security Systems, Inc.;

      2.    Based on my knowledge, this report does not contain any untrue
            statement of a material fact or omit to state a material fact
            necessary to make the statements made, in light of the circumstances
            under which such statements were made, not misleading with respect
            to the period covered by this report;

      3.    Based on my knowledge, the financial statements, and other financial
            information included in this report, fairly present in all material
            respects the financial condition, results of operations and cash
            flows of the registrant as of, and for, the periods presented in
            this report;

      4.    The registrant's other certifying officer and I are responsible for
            establishing and maintaining disclosure controls and procedures (as
            defined in Exchange Act Rules 13a-15(e) and 15d-15(e)) for the
            registrant and have:

            a) designed such disclosure controls and procedures, or caused such
            disclosure controls and procedures to be designed under our
            supervision, to ensure that material information relating to the
            registrant, including its consolidated subsidiaries, is made known
            to us by others within those entities, particularly during the
            period in which this report is being prepared;

            b) evaluated the effectiveness of the registrant's disclosure
            controls and procedures and presented in this report our conclusions
            about the effectiveness of the disclosure controls and procedures as
            of the end of the period covered by this report based on such
            evaluation; and

            c) disclosed in this report any change in the registrant's internal
            control over financial reporting that occurred during the
            registrant's most recent fiscal quarter that has materially
            affected, or is reasonably likely to materially affect, the
            registrant's internal control over financial reporting; and

      5.    The registrant's other certifying officer and I have disclosed,
            based on our most recent evaluation of internal control over
            financial reporting, to the registrant's auditors and the audit
            committee of registrant's board of directors (or persons performing
            the equivalent functions):

            a) all significant deficiencies and material weaknesses in the
            design or operation of internal control over financial reporting
            which are reasonably likely to adversely affect the registrant's
            ability to record, process, summarize and report financial
            information; and

            b) any fraud, whether or not material, that involves management or
            other employees who have a significant role in the registrant's
            internal control over financial reporting.

                Date: November 1, 2004             /s/ Thomas E. Noonan
                                                  ------------------------------
                                                  Thomas E. Noonan
                                                  Chairman, President and
                                                  Chief Executive Officer