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                                                                     Exhibit 5.1

                     [LOCKE LIDDELL & SAPP LLP LETTERHEAD]

                                                   Direct Number: (214) 740-8675
                                               email: jmcknight@lockeliddell.com

                               September 28, 2001

Carreker Corporation
4055 Valley View Lane
Suite 1000
Dallas, Texas  75244

Ladies and Gentlemen:

    We have acted as counsel for Carreker Corporation (formerly
Carreker-Antinori, Inc.), a Delaware corporation (the "Company"), in
connection with the registration under the Securities Act of 1933, as amended
(the "Securities Act"), pursuant to a Registration Statement on Form S-8 (the
"Registration Statement"), of an additional One Million Eight Hundred
Thousand (1,800,000) shares of Common Stock, par value $.01 per share (the
"Additional LTIP Shares"), of the Company under the Company's Third Amended
and Restated Carreker-Antinori, Inc. 1994 Long Term Incentive Plan (the
"LTIP"), and an additional One Hundred Thousand (100,000) shares of Common
Stock, par value $.01 per share, of the Company under the Company's Amended
and Restated Director Stock Option Plan (the "Director Plan") (the
"Additional Director Shares")(the Additional LTIP Shares and the Additional
Director Shares are collectively referred to herein as the "Additional
Shares").

    In connection with this opinion, we have examined the Registration
Statement, the Company's Certificate of Incorporation and Bylaws, each as
amended to date, and such other documents, records, certificates, memoranda
and other instruments as we deem relevant or 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.

    Based upon our examination of such documents and the investigations of
such matters of law as we have deemed relevant or necessary in rendering this
opinion, we hereby advise you that we are of the opinion that on the basis of
the foregoing, assuming, with respect to the Additional Common Shares issued
under the LTIP and the Director Plan, (A) the receipt of proper consideration
for the issuance thereof in excess of the par value thereof, (B) the
availability of a sufficient number of shares of Common Stock authorized by
the Company's Certificate of Incorporation then in effect, (C) compliance
with the terms of any agreement entered into in connection with any options
or restricted stock under the LTIP, and (D) no change occurs in the
applicable law or the pertinent facts, the shares of Common Stock purchasable
upon the exercise of any option granted under or issued upon the awarding of
any restricted stock under the LTIP or the Director Plan will upon issuance
be duly authorized and validly issued, fully paid and nonassessable.

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    We are expressing the opinions above as members of the Bar of the State
of Texas and we express no opinion as to any laws other than the laws of the
State of Texas and, to the extent relevant to the opinions herein, the
General Corporation Law of the State of Delaware. You should be aware that we
are not admitted to the practice of law in the State of Delaware, and any
opinion herein as to the laws of such state is based solely on the most
recent unofficial compilation of the corporate statutes of the State of
Delaware available to us.

    We consent to the filing of this opinion as Exhibit 5.1 to the
Registration Statement filed by the Company with the Securities and Exchange
Commission for the registration under the Securities Act of an additional One
Million Eight Hundred Thousand (1,800,000) shares of Common Stock of the
Company covered by the LTIP and One Hundred Thousand (100,000) shares of
Common Stock of the Company covered by the Director Plan. By so consenting,
we do not thereby admit that our firm's consent is required by Section 7 of
the Securities Act.

                                       Very truly yours,


                                       LOCKE LIDDELL & SAPP LLP

                                       /s/ John B. McKnight
                                       ------------------------
                                       John B. McKnight