Exhibit 8.1


                  [Letterhead of Gibson, Dunn, & Crutcher LLP]
                                  May 28, 2002

(213) 229-7000                                                     C 18861-00076

Cadence Design Systems, Inc.
2655 Seely Avenue
San Jose, California

         Re:      Registration Statement on Form S-4

Ladies and Gentlemen:

         We have acted as counsel to Cadence Design Systems, Inc., a Delaware
corporation ("PARENT"), in connection with the preparation and execution of the
Agreement and Plan of Merger (the "MERGER AGREEMENT"), dated as of April 24,
2002, by and among Parent, Zodiac Acquisition, Inc., a Delaware corporation and
a wholly owned subsidiary of Parent, and Simplex Solutions, Inc., a Delaware
corporation (the "COMPANY"). At your request, we have examined the form of
Registration Statement on Form S-4 filed with the U.S. Securities and Exchange
Commission, including the Proxy Statement/Prospectus that forms a part thereof
(the "REGISTRATION STATEMENT") in connection with the registration of the shares
of Parent's common stock to be issued to the stockholders of Company upon
consummation of the Merger. Capitalized terms used but not defined herein have
the meanings ascribed thereto in the Merger Agreement.

         You have requested our opinion regarding the accuracy of the federal
income tax matters described in the Registration Statement under the caption
"Material U.S. Federal Income Tax Consequences of the Merger".

         In rendering this opinion, we have reviewed (without any independent
investigation) the Merger Agreement, the Registration Statement and such other
documents as we have deemed necessary or appropriate. We have relied upon the
truth and accuracy at all relevant times of the facts, statements, covenants,
representations and warranties contained in the Merger Agreement and the
Registration Statement, and have assumed that the Merger will be consummated in


Cadence Design Systems, Inc.
May 28, 2002
Page 3


accordance with the Merger Agreement and without breach or waiver of any
material provision thereof. We have also assumed the authenticity of original
documents submitted to us, the conformity to the originals of documents
submitted to us as copies, and the due and valid execution and delivery of all
such documents where due execution and delivery are a prerequisite to the
effectiveness thereof.

         Based upon the foregoing, it is our opinion that the discussion in the
Registration Statement, under the caption "Material U.S. Federal Income Tax
Consequences of the Merger," to the extent it constitutes descriptions of legal
matters or legal conclusions is accurate in all material respects.

         This opinion represents our best judgment regarding the application of
federal income tax laws under the Internal Revenue Code of 1986, as amended,
existing judicial decisions, administrative regulations and published rulings
and procedures. Our opinion is not binding upon the Internal Revenue Service or
the courts, and there is no assurance that the Internal Revenue Service will not
successfully assert a contrary position. This opinion is being delivered prior
to the Effective Time and therefore is prospective and dependent on future
events. No assurance can be given that future legislative, judicial or
administrative changes, on either a prospective or retroactive basis, or future
factual developments, would not adversely affect the accuracy of the conclusions
stated herein. We undertake no responsibility to advise you of any new
developments in the facts or in the application or interpretation of the federal
income tax laws. Furthermore, in the event any one of the statements,
representations, warranties or assumptions upon which we have relied to issue
this opinion is incorrect, our opinion might be adversely affected and may not
be relied upon.

         This opinion addresses only the matters described above, and does not
address any other federal, state, local or foreign tax consequences that may
result from the Merger.

         We hereby consent to the use of this opinion as an Exhibit to the
Registration Statement and to the use of our name under the caption "Material
U.S. Federal Income Tax Consequences of the Merger" in the Registration
Statement. In giving such consent, we do not thereby admit that we are in the
category of persons whose consent is required under Section 7 of the Securities
Act, nor do we thereby admit that we are experts with respect to any part of
such Registration Statement within the meaning of the term "experts" as used in
the Securities Act.

                                          Very truly yours,



                                          GIBSON, DUNN & CRUTCHER LLP

PSI/DA