Exhibit 31.1

CERTIFICATION

I, George T. Shaheen, certify that:

1. I have reviewed this quarterly report on Form 10-Q of Siebel 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(s) 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)) and internal control over financial reporting (as defined in Exchange Act
Rules 13a-15(f) and 15d-15(f)) 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) Designed such internal control over financial reporting, or caused such
internal control over financial reporting to be designed under our supervision, to provide
reasonable assurance regarding the reliability of financial reporting and the preparation of
financial statements for external purposes in accordance with generally accepted accounting
principles;

c) 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

d) 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 (the
registrant's fourth fiscal quarter in the case of an annual report) 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(s) 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 the 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: August 5, 2005 |                         |                      
|-------------------- | ----------------------- | ---------------------
|                     | By:                     | /s/ George T. Shaheen
|                     | George T. Shaheen      
|                     | Chief Executive Officer

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