Exhibit 5.1

                      [LETTERHEAD OF COOLEY GODWARD LLP]


November 30, 2000



Siebel Systems, Inc.
2207 Bridgepointe Parkway
San Mateo, CA 94404

Re:  Form S-8

Ladies and Gentlemen:

You have requested our opinion with respect to certain matters in connection
with the filing by Siebel Systems, Inc. (the "Company") of a Registration
Statement on Form S-8 (the "Registration Statement") with the Securities and
Exchange Commission covering the offering of up to 1,550,573 shares of the
Company's Common Stock, $.001 par value (the "Shares"), issuable upon the
exercise of stock options originally granted under the Amended and Restated
Share Compensation Plan (the "Janna Plan") of Janna Systems Inc. ("Janna") and
assumed by the Company pursuant to the Arrangement Agreement, dated as of
September 11, 2000, among the Company, Janna Nova Scotia Sub Company (formerly
3045856 Nova Scotia Company), Siebel Janna Arrangement, Inc. (formerly 2000066
Ontario Inc.) and Janna (the "Arrangement Agreement").

In connection with this opinion, we have examined the Arrangement Agreement, the
Registration Statement and related Prospectus, the Company's Amended and
Restated Certificate of Incorporation and Bylaws, each as amended to date, and
such other documents, records, certificates, memoranda and other instruments as
we deem necessary as a basis for this opinion.  We have assumed the genuineness
and authenticity of all documents submitted to us as originals, the conformity
to originals of all documents submitted to us as copies thereof and the due
execution and delivery of all documents where due execution and delivery are a
prerequisite to the effectiveness thereof.

Our opinion is expressed only with respect to the federal laws of the United
States of America, the General Corporation Law of the State of Delaware and the
laws of the State of California. We express no opinion as to whether the laws of
any particular jurisdiction other than those identified above are applicable to
the subject matter hereof.

On the basis of the foregoing, and in reliance thereon, we are of the opinion
that the Shares, when sold and issued in accordance with the Janna Plan, the
Registration Statement and related Prospectus, will be validly issued, fully
paid, and nonassessable (except as to shares issued pursuant to certain deferred
payment arrangements, if any, which will be fully paid and nonassessable when
such deferred payments are made in full).

We consent to the filing of this opinion as an exhibit to the Registration
Statement.

Very truly yours,

Cooley Godward LLP


By:  /s/ Vincent P. Pangrazio
     ----------------------------
     Vincent P. Pangrazio

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