Exhibit 5.1

		140 Scott Drive
		Menlo Park, California 94025
		Tel: +1.650.328.4600 Fax: +1.650.463.2600
		www.lw.com
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Calix, Inc.

1035 N. McDowell Blvd.

Petaluma, CA 94954

Ladies and Gentlemen:

We have acted as special counsel to Calix, Inc., a Delaware corporation (the “Company”), in connection with the registration statement on Form S-1 filed by the Company with the Securities and Exchange Commission (the “Commission”) on November 20, 2009 (File No. 333-163252), as amended (the “Registration Statement”), under the Securities Act of 1933, as amended (the “Act”). The Registration Statement relates to the registration of up to 7,278,271 shares of common stock of the Company, par value $0.025 per share, up to 4,166,666 shares of which are being offered by the Company (the “Company Primary Shares”), up to 2,162,266 shares of which are being offered by certain stockholders of the Company (the “Selling Stockholder Shares”) and up to 949,339 shares of which may be purchased by the underwriters pursuant to an option to purchase additional shares granted by the Company (the “Company Option Shares,” and together with the Company Primary Shares, the “Company Shares”). The Company Shares and the Selling Stockholder Shares are referred to herein collectively as the “Shares.” This opinion is being furnished in connection with the requirements of Item 601(b)(5) of Regulation S-K under the Act, and no opinion is expressed herein as to any matter pertaining to the contents of the Registration Statement or related prospectus (the “Prospectus”), other than as expressly stated herein with respect to the issue of the Shares.

As such counsel, we have examined such matters of fact and questions of law as we have considered appropriate for purposes of this letter. With your consent, we have relied upon certificates and other assurances of officers of the Company and others as to factual matters without having independently verified such factual matters.

We are opining herein as to General Corporation Law of the State of Delaware (the “DGCL”), and we express no opinion with respect to any other laws.

Subject to the foregoing and the other matters set forth herein, as of the date hereof:

March 23, 2010

Page 2

1. When the Company Shares shall have been duly registered on the books of the transfer agent and registrar therefor in the name or on behalf of the purchasers, and have been issued by the Company against payment therefor in the circumstances contemplated by the form of underwriting agreement most recently filed as an exhibit to the Registration Statement, the issue and sale of the Company Shares will have been duly authorized by all necessary corporate action of the Company, and the Company Shares will be validly issued, fully paid and nonassessable. In rendering the foregoing opinion, we have assumed that the Company will comply with all applicable notice requirements regarding uncertificated shares provided in the DGCL.

2. The Selling Stockholder Shares have been duly authorized by all necessary corporate action of the Company and are validly issued, fully paid and nonassessable.

This opinion is for your benefit in connection with the Registration Statement and may be relied upon by you and by persons entitled to rely upon it pursuant to the applicable provisions of the Act. We consent to your filing this opinion as an exhibit to the Registration Statement and to the reference to our firm in the Prospectus under the heading “Legal Matters.” In giving such consent, we do not thereby admit that we are in the category of persons whose consent is required under Section 7 of the Act or the rules and regulations of the Commission thereunder.

Very truly yours,

/s/ Latham & Watkins LLP