[WILSON SONSINI GOODRICH & ROSATI LETTERHEAD]



                                  July 14, 2004


Nanosys, Inc.
2625 Hanover Street
Palo Alto, California 94304


     RE:  REGISTRATION STATEMENT ON FORM S-1


Ladies and Gentlemen:

     We have examined the Registration Statement on Form S-1 filed with the
Securities and Exchange Commission on April 22, 2004, as hereafter amended or
supplemented (the "Registration Statement"), in connection with the registration
under the Securities Act of 1933, as amended, of 7,187,500 shares of Common
Stock (the "Shares") of Nanosys, Inc. (the "Company"). The Shares, which include
up to 937,500 shares of Common Stock issuable pursuant to an over-allotment
option granted to the underwriters, are to be sold to the underwriters as
described in such Registration Statement for sale to the public or issued to the
Representatives of the underwriters. As your counsel in connection with this
transaction, we have examined the proceedings proposed to be taken in connection
with said sale and issuance of the Shares.

     Additionally, as your counsel, we have examined originals or copies,
certified or otherwise identified to our satisfaction, of such documents,
corporate records, certificates of public officials and other instruments as we
have deemed necessary for the purposes of rendering this opinion. In our
examination, we have assumed the genuineness of all signatures, the authenticity
of all documents submitted to us as originals and the conformity with the
originals of all documents submitted to us as copies.

     Based on the foregoing, it is our opinion that the Shares, when issued and
sold in the manner described in the Registration Statement and in accordance
with the resolutions adopted by the Board of Directors of the Company, will be
legally and validly issued, fully paid and nonassessable.

     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, including the Prospectus constituting a part thereof,
and any amendments thereto.


                                     Sincerely,

                                     WILSON SONSINI GOODRICH & ROSATI
                                     Professional Corporation

                                     /s/ Wilson Sonsini Goodrich & Rosati