EXHIBIT 5.01


September 26, 1996



Integrated Systems, Inc.
201 Moffett Park Drive
Sunnyvale, California 94089

Gentlemen/Ladies:

         At your request,  we have examined the  Registration  Statement on Form
S-8 (the  "Registration  Statement")  to be filed by you with the Securities and
Exchange  Commission  on or about  September  27,  1996 in  connection  with the
registration  under the Securities  Act of 1933, as amended,  of an aggregate of
69,033  shares of your  Common  Stock  (the  "Common  Stock")  to be sold by you
pursuant to stock options granted under the Epilogue Technology Corporation 1994
Stock Option Plan and assumed by you (the  "Options").  The Options were assumed
by you pursuant to the terms of an Agreement and Plan of Reorganization dated as
of June 28, 1996 (the  "Reorganization  Agreement") by and among you, ISI Merger
Corporation,  a Delaware  corporation  and your  wholly  owned  subsidiary,  and
Epilogue  Technology  Corporation,  a  Delaware  corporation,  and  the  related
Agreement of Merger dated July 29, 1996, which together with the  Reorganization
Agreement  effectuated a merger of ISI Merger Corporation with and into Epilogue
Technology Corporation.

         As your  counsel,  we have  examined  the  proceedings  taken by you in
connection with the assumption of the Options to purchase your Common Stock.

         It is our opinion that the number of shares of Common Stock that may be
issued and sold by you pursuant to the Options as indicated  above,  when issued
and  sold in the  manner  referred  to in the  Prospectus  associated  with  the
Registration  Statement,  the Epilogue Technology  Corporation Stock Option Plan
and the Options, will be legally issued, fully paid and non-assessable.

         We consent to the use of this opinion as an exhibit to the Registration
Statement  and  further  consent  to  all  references  to  us,  if  any,  in the
Registration Statement and any amendments thereto.

                                      Very truly yours,



                                      /s/ Fenwick & West LLP



                                      Fenwick & West LLP