Exhibit 5.1


                       [GREENBERG TRAURIG, LLP LETTERHEAD]


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


January 31, 2005

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

Re:      Acacia Research Corporation Registration Statement on Form S-3 to be
         Filed with the Securities and Exchange Commission on February 1, 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 shareholders (the "Selling Shareholders") 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 Shareholders.


Moreover, our representation of the Company is necessarily limited to such
matters referred to us from time to time by representatives of the Company.
Accordingly, we do not have and you should not infer from our representation of
the Company in this particular instance that we have any knowledge of the
Company's affairs or transactions other than as expressly set forth in this
opinion letter.


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).






Board of Directors
January 31, 2005
Page 2


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 Shareholders 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.


This opinion letter is governed by, and shall be interpreted in accordance with,
the Legal Opinion Accord (the "Accord") of the ABA Section of Business Law
(1991). As a consequence, it is subject to a number of qualifications,
exceptions, definitions, limitations on coverage and other limitations, all as
more particularly described in the Accord, and this opinion letter should be
read in conjunction therewith.


The opinions expressed herein are written as of and relate solely to the date
hereof and are rendered exclusively for your benefit 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


Greenberg Traurig, LLP