Exhibit 5.1



                     [LETTERHEAD OF CAHILL GORDON & REINDEL]



                                                               November 28, 2000

Investment Technology Group, Inc.
380 Madison Avenue
4th Floor
New York, New York  10017

Re:  Form S-8 Relating to the ITG Australia Share Plan
     -------------------------------------------------

Ladies and Gentlemen:

     We have acted as counsel to Investment Technology Group, Inc., a Delaware
corporation (the "Company"), in connection with the preparation of the
Registration Statement on Form S-8 (the "Registration Statement") to be filed
with the Securities and Exchange Commission (the "Commission") under the
Securities Act of 1933, as amended (the "Securities Act"), relating to 19,353
shares of the Company's common stock (the "Registered Shares"), $.01 par value
per share, offered under the ITG Australia Employee Share Plan (the "Plan").

     In connection with this opinion, we have examined the Registration
Statement, the Certificate of Incorporation and Bylaws of the Company, certain
of the Company's corporate proceedings as reflected in its minute books, the
Plan and such other records as we have deemed relevant. In our examinations, 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. As to factual matters, we have relied
on certificates and other representations and warranties of officers of the
Company and its subsidiaries and certificates and other documents of or provided
by governmental officials as we have deemed necessary or desirable.

     In our opinion, the Registered Shares, when issued and sold by the Company
in accordance with the Plan, will be duly authorized, validly issued, fully paid
and non-assessable.

     In rendering this opinion, we express no opinion as to the laws of any
jurisdiction other than the laws of the United States, the laws of the State of
New York and the General Corporation Law of the State of Delaware.

     We hereby consent to the filing of this opinion as Exhibit 5 to the
Registration Statement. In giving such opinion and consent, we do not thereby
admit that we are acting within the category of persons whose consent is
required under Section 7 of the Securities Act or the rules and regulations of
the Commission thereunder.

                                              Very truly yours

                                              /s/ Cahill Gordon & Reindel