Exhibit 5.1

                                  July 5, 2000

Nanometrics Incorporated
310 DeGuigne Drive
Sunnyvale, California 94086

               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 6, 2000 (the
"Registration   Statement")  in  connection  with  the  registration  under  the
Securities  Act of 1933,  as amended,  of 150,000  shares of your  Common  Stock
reserved for issuance  under your  Employee  Stock  Purchase  Plan (the "ESPP"),
1,250,000  shares of your Common Stock  reserved  for  issuance  under your 2000
Employee Stock Option Plan (the "Employee Stock Option Plan") and 250,000 shares
of your Common Stock reserved for issuance under your 2000 Director Stock Option
Plan (the "Director Stock Option Plan"). As your legal counsel, we have examined
the proceedings taken and are familiar with the proceedings proposed to be taken
in connection  with the issuance and sale of such shares of your Common Stock to
be issued under the ESPP, the Employee Stock Option Plan, and the Director Stock
Option Plan.

         It is our opinion that,  upon completion of the actions being taken, or
contemplated  by us as your  counsel to be taken by you prior to the issuance of
such shares of your Common Stock  pursuant to the  Registration  Statement,  the
ESPP,  the Employee  Stock Option Plan,  and the Director  Stock Option Plan and
upon completion of the actions being taken in order to permit such  transactions
to be carried out in accordance  with the securities  laws of the various states
where  required,  such shares of your  Common  Stock 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