[LETTERHEAD OF WILSON SONSINI GOODRICH & ROSATI]

                                                                     Exhibit 5.1

                                  July 1, 2002

Nanometrics Incorporated
1550 Buckeye Drive
Milpitas, CA 95035

     Re: Registration Statement on Form S-8

Ladies and Gentlemen:

         We have examined the Registration  Statement on Form S-8 to be filed by
you with the  Securities  and Exchange  Commission on or about July 1, 2002 (the
"Registration   Statement")  in  connection  with  the  registration  under  the
Securities  Act of 1933,  as  amended,  of  1,200,000  additional  shares of the
Company's  Common Stock with no par value per share (the  "Shares")  reserve and
for issuance under the Nanometrics  Incorporated 2000 Employee Stock Option Plan
(the "Plan").  As legal counsel for Nanometrics  Incorporated,  we have examined
the proceedings taken and are familiar with the proceedings proposed to be taken
by you in  connection  with the issuance and sale of the Shares  pursuant to the
Plan.

         It is our opinion  that the Shares,  when issued and sold in the manner
described in the Plan and pursuant to the agreement that  accompanies each grant
under  the  Plan,   will  be  legally  and  validly   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 the use of our name wherever appearing in the
Registration Statement and any amendments thereto.

                                            Very truly yours,

                                            WILSON SONSINI GOODRICH & ROSATI
                                            Professional Corporation


                                            /s/ WILSON SONSINI GOODRICH & ROSATI