EXHIBIT 5.1
October 16, 2003

PDF Solutions, Inc.
333 West San Carlos Street, Suite 700
San Jose, California 95110

         Re:      PDF Solutions, Inc.
                  Registration Statement on Form S-8

Ladies and Gentlemen:

         At your request, we are rendering this opinion in connection with the
Registration Statement on Form S-8 (the "Registration Statement") filed by you
with the Securities and Exchange Commission (the "Commission") on or about
October 16, 2003 in connection with the registration under the Securities Act of
1933, as amended, of a total of 926,695 shares of your Common Stock (the "Common
Shares") reserved for issuance under the 2001 Stock Option/Stock Issuance Plan
assumed by you in connection with your acquisition of all of the issued and
outstanding shares of capital stock of IDS Software Systems, Inc.

         We have examined instruments, documents, and records which we deemed
relevant and necessary for the basis of our opinion hereinafter expressed. In
such examination, we have assumed the following: (a) the authenticity of
original documents and the genuineness of all signatures; (b) the conformity to
the originals of all documents submitted to us as copies; and (c) the truth,
accuracy, and completeness of the information, representations, and warranties
contained in the records, documents, instruments, and certificates we have
reviewed.

         Based on such examination, we are of the opinion that the Common
Shares, when issued and sold as described in the Registration Statement, will be
legally issued, fully paid and nonassessable.

         We hereby consent to the filing of this opinion as an exhibit to the
above-referenced Registration Statement and to the use of our name wherever it
appears in said Registration Statement, including the Prospectus constituting a
part thereof, as originally filed or as subsequently amended or supplemented. In
giving such consent, we do not consider that we are "experts" within the meaning
of such term as used in the Securities Act of 1933, as amended, or the rules and
regulations of the Commission issued thereunder, with respect to any part of the
Registration Statement, including this opinion as an exhibit or otherwise.

Very truly yours,

/s/ ORRICK, HERRINGTON & SUTCLIFFE LLP