EXHIBIT 5.1

                   [Letterhead of Gibson, Dunn & Crutcher LLP]

                                  July 2, 2002

Cadence Design Systems, Inc.
2655 Seely Avenue, Building 5
San Jose, CA 95134

        Re:     Post-Effective Amendment No. 1 on Form S-8 to Form S-4
                Registration Statement of Cadence Design Systems, Inc.

Ladies and Gentlemen:

        We refer to the post-effective amendment No. 1 on Form S-8 to Form S-4
registration statement ("Registration Statement"), under the Securities Act of
1933, as amended (the "Securities Act"), filed by Cadence Design Systems, Inc.,
a Delaware corporation (the "Company"), with respect to the proposed offering by
the Company of up to 4,527,600 shares (the "Shares") of the common stock of the
Company, $0.01 par value per share (the "Common Stock"), subject to issuance by
the Company upon exercise of options granted under the Simplex Solutions, Inc.
1995 Stock Plan, Simplex Solutions, Inc. 2001 Incentive Stock Plan, Simplex
Solutions, Inc. 2002, Nonstatutory Stock Option Plan, or the Altius Solutions,
Inc. 1999 Plan (the "Plans") assumed by the Company pursuant to the terms of the
Agreement and Plan of Merger, dated as of April 24, 2002, among the Company,
Zodiac Acquisition, Inc., a Delaware corporation and wholly-owned subsidiary of
the Company, and Simplex Solutions, Inc., a Delaware corporation (the "Merger
Agreement").

        We have examined the originals or certified copies of such corporate
records, certificates of officers of the Company and/or public officials and
such other documents, and have made such other factual and legal investigations,
as we have deemed relevant and necessary as the basis for the opinions set forth
below. In such examination, we have assumed the genuineness of all signatures,
the authenticity of all documents submitted to us as originals, the conformity
to original documents of all documents submitted to us as conformed or
photostatic copies and the authenticity of the originals of such copies.

        Based on our examination described above, subject to the assumptions
stated above and relying on the statements of fact contained in the documents
that we have examined, we are of the opinion that (i) the issuance by the
Company of the Shares has been duly authorized and (ii) when issued in
accordance with the terms of the Plans and the Merger Agreement, the Shares will
be duly and validly issued, fully paid and non-assessable shares of Common
Stock.

        We are admitted to practice in the State of California, and are not
admitted to practice in the State of Delaware. However, for the limited purpose
of our opinions set forth above, we are generally familiar with the General
Corporation Law of the State of Delaware (the "DGCL") as presently in effect and
have made such inquiries as we consider necessary to render this opinion with
respect to a Delaware corporation. This opinion letter is limited to the laws of
the State of California and, to the limited extent set forth above, the DGCL, as
such laws presently exist and to the facts as they presently exist. We express
no opinion with respect to the effect or applicability of the laws of any other
jurisdiction. We assume no obligation to revise or supplement this opinion
letter should the laws of such jurisdictions be changed after the date hereof by
legislative action, judicial decision or otherwise.

        We hereby consent to the filing of this opinion as an exhibit to the
Registration Statement. In giving this consent, we do not admit that we are
within the category of persons whose consent is required under Section 7 of the
Securities Act or the General Rules and Regulations of the Securities and
Exchange Commission.


                                        Very truly yours,

                                        /s/ GIBSON, DUNN & CRUTCHER LLP