Exhibit 5.1


                       [GREENBERG TRAURIG, LLP LETTERHEAD]

Raymond A. Lee
Tel. 714.708.6511
Fax 701.708.6501
LeeR@gtlaw.com

April 28, 2005

Board of Directors
Acacia Research Corporation.
500 Newport Center Drive
7th Floor
Newport Beach, CA 92660

Re:      Acacia Research Corporation Amended Registration Statement on Form
         S-3/A to be Filed with the Securities and Exchange Commission on or
         about May 2, 2005

Gentlemen:

This opinion is furnished to you in connection with the above-referenced
registration statement on Form S-3 (the "Registration Statement"), to be filed
with the Securities and Exchange Commission ("SEC") under the Securities Act of
1933, as amended (the "Act"), for the registration and resale of up to 3,938,832
shares of Acacia Research-Acacia Technologies common stock (the "Securities") of
Acacia Research Corporation, a Delaware corporation (the "Registrant") on behalf
of the selling stockholders (the "Selling Stockholders") as described in the
Registration Statement.

We have acted as counsel for the Registrant in connection with the Registration
Statement. For purposes of this opinion, we have assumed, without independent
verification or investigation, that each instrument has been duly and validly
authorized, executed, and delivered by each of the parties thereto, the
genuineness of all of all signatures, the authenticity of all documents
submitted to us as originals, and conformity with the authentic originals of all
documents submitted to us as copies. In rendering the opinions included herein,
we have relied upon the factual representations and warranties made by the
Registrant and the Selling Stockholders.

We express no opinion as to any jurisdiction other than federal securities laws
and the Delaware General Corporation Law (including, to the extent applicable,
Delaware statutory and constitutional provisions and reported case law).

Based on the foregoing, and subject to the qualifications and assumptions stated
herein, we are of the opinion that:

         1. The Securities are validly issued, fully paid and non-assessable,
         and will remain so upon resale under the Registration Statement in
         accordance with the Act and the rules and regulations promulgated
         thereunder.

         2. Provided that the Registration Statement is declared effective and
         remains in effect, and that the provisions of the Act and the rules and
         regulations promulgated thereunder, including the prospectus delivery
         requirement, are complied with by the Selling Stockholders and their
         respective brokers, the restrictive legends on the Securities may be
         removed in accordance with Rule 415 under the Act.

The opinions contained in this opinion letter merely constitute expressions of
my reasoned professional judgment regarding the matters of law addressed herein
and neither are intended nor should they be construed as a prediction or
guarantee that any court or other public or governmental authority will reach
any particular result or conclusion as to the matters of law addressed herein.



Board of Directors
April 28, 2005
Page 2

The opinions expressed herein are written as of and relate solely to the date
hereof and are rendered exclusively for the benefit of the Company and its
stockholders in connection with the Registration. We assume no obligation to
advise you of facts, circumstances, events or developments which hereafter may
be brought to our attention and which may alter, affect or modify the opinions
expressed herein.


We hereby consent to use this opinion as an exhibit to the Registration
Statement and to the use of our name therein and in the related prospectus under
the caption "Legal Matters." In giving such consent, we do not thereby admit
that we come within the category of persons whose consent is required under
Section 7 of the Act, or the Rules and Regulations of the SEC thereunder.


It is understood that this opinion is to be used only in connection with the
offer and sale of the Securities while the Registration Statement is in effect.


Sincerely,


/s/ GREENBERG TRAURIG
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Greenberg Traurig, LLP