Exhibit 5 


                                BAKER & McKENZIE
                                Attorneys at Law

                              One Prudential Plaza
                            130 East Randolph Drive
                             Chicago, Illinois 60601
                                February 27, 1997

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

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

Gentlemen:

     We have acted as your counsel in  connection  with the  registration,  on a
Registration  Statement  on Form S-8 (the  "Registration  Statement")  under the
Securities  Act of 1933,  as  amended,  of (i) options  ("Options")  to purchase
1,050,000  shares of the  Company's  Class A Common  Stock,  $.005 par value per
share (the "Stock"),  to be issued by the Company pursuant to the Company's 1997
Management  Stock Option Plan (the  "Plan"),  and (ii) the  1,050,000  shares of
Stock underlying the Options. We have reviewed the Registration  Statement,  the
charter  and  by-laws  of the  Company,  corporate  proceedings  of the Board of
Directors  relating to the  issuance of the  Options  and  underlying  shares of
Stock, and such other documents,  corporate  records and questions of laws as we
have deemed necessary to the rendering of the opinions  expressed  below.

     Based upon the  foregoing,  we are of the opinion  that the Options and the
1,050,000 shares of Stock underlying the Options to be issued by the Company, as
described in the Plan,  will be legally  issued,  fully paid and  non-assessable
when issued and paid for in the manner contemplated in the Plan.

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

                                             Very Truly Yours,

                                             Baker & McKenzie