EXHIBIT 5.1
                                BAKER & MCKENZIE
                                Attorneys at Law

                            2300 Trammell Crow Center
                                2001 Ross Avenue
                               Dallas, Texas 75201


                                February 27, 2002


Board of Directors
Transaction Systems Architects, Inc.
224 South 108th Avenue
Omaha, Nebraska 68154

Re:  Transaction Systems Architects, Inc. (the "Company")

Gentlemen:

         The Company has filed with the Securities and Exchange Commission (the
"Commission") a registration statement (the "Registration Statement") on Form
S-8 under the Securities Act of 1933, as amended (the "Act"). The Registration
Statement covers (a) 250,000 shares of Class A Common Stock, par value $.005 per
share, of the Company (the "Common Stock"), which shares shall be issued
pursuant to the Transaction Systems Architects, Inc. 2002 Non-Employee Stock
Option Plan (the "2002 Stock Option Plan"), and (b) such additional shares of
Common Stock as may become issuable pursuant to the anti-dilution provisions of
the 2002 Stock Option Plan (such shares collectively referred to as the
"Securities").

         We have acted as counsel to the Company in connection with the
preparation and filing of the Registration Statement. In rendering this opinion
we have examined such corporate records, documents and instruments of the
Company and such certificates of public officials, have received such
representations from officers of the Company, and have reviewed such questions
of law as in our judgment are necessary, relevant or appropriate to enable us to
render the opinion expressed below. In such examination, we have assumed the
genuineness of all signatures, the authenticity of all corporate records,
documents and instruments submitted to us as originals, the conformity to
original documents of all documents submitted to us as conformed, certified or
photostatic copies thereof, and the authenticity of the originals of such
conformed, certified or photostatic copies.

         Based upon such examination and review and upon representations made to
us by officers of the Company, we are of the opinion that upon issuance and
delivery of the Securities in accordance with the applicable terms and
conditions of the 2002 Stock Option Plan and upon receipt by the Company of the
full consideration for the Securities as determined pursuant to the 2002 Stock
Option Plan, the Securities will be legally issued, fully paid and
nonassessable.

         This firm consents to the filing of this opinion as an exhibit to the
Registration Statement. In giving such consent, we do not admit that we come
within the category of persons whose consent is required by Section 7 of the Act
or the rules and regulations of the Commission thereunder.

                                                Very truly yours,



                                                Baker & McKenzie