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                                                                    EXHIBIT 5.1

                   [Letterhead of Gibson, Dunn & Crutcher LLP]

                                  May 10, 2004

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

         Re:      Registration Statement on Form S-8 of Cadence Design Systems,
                  Inc.

Ladies and Gentlemen:

                  We refer to the registration statement on Form S-8 (the
"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
47,970 shares (the "Shares") of its common stock, $0.01 par value per share (the
"Common Stock"), subject to issuance by the Company upon exercise of options
granted under the QDA, Inc. 2003 Stock Option/Stock Issuance Plan (the "Plan").
The options issuable under the Plan were assumed by the Company pursuant to the
terms of that certain Agreement and Plan of Merger, dated as of December 19,
2003, by and among the Company, Project Mendocino, Inc., a California
corporation and wholly owned subsidiary of the Company, QDA, Inc., a California
corporation, and David Marple, as Shareholder Agent.

                  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 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 mentioned 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 Plan, 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 DGCL, as it presently exists 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

