Exhibits 5.1 and 23.2

August 22, 2006

Marsh & McLennan Companies, Inc.

1166 Avenue of the Americas

New York, New York 10036-2774

Ladies and Gentlemen:

We have acted as counsel for Marsh & McLennan Companies, Inc., a Delaware company (the “Company”) in connection with the Registration Statement on Form S-3ASR (the “Registration Statement”) filed by the Company with the Securities and Exchange Commission pursuant to the Securities Act of 1933, as amended (the “Securities Act”), for the registration of the sale from time to time of (a) shares of common stock, par value $1.00 per share (the “Common Stock”) of the Company, together with Preferred Stock Purchase Rights (the “Rights”) attached to the shares of Common Stock and representing rights to subscribe for and to purchase one six-hundredth of a share of preferred stock of the Company, issued pursuant to the Amended and Restated Rights Agreement dated as of January 20, 2000, as amended (the “Rights Agreement”); (b) shares of preferred stock, par value $1.00 per share (the “Preferred Stock”) of the Company; (c) the Company’s senior debt securities and subordinated debt securities (collectively, the “Debt Securities”), which may be issued pursuant to an indenture for senior debt securities (the “Senior Indenture”) to be entered into between the Company and The Bank of New York, as trustee (the “Trustee”) and an indenture for subordinated debt securities to be entered into between the Company and the Trustee (the “Subordinated Indenture and together with the Senior Indenture, the “Indentures,” the forms of which have been filed as exhibits to the Registration Statement); (d) depositary shares representing interests in Preferred Stock (“Depositary Shares”), which may be issued pursuant to a deposit agreement to be entered into between the Company and the depositary to be named therein (the “Deposit Agreement”); (e) warrants of the Company (the “Warrants”), which may be issued pursuant to a warrant agreement to be entered into between the Company and the warrant agent to be named therein (the “Warrant Agreement”); (f) purchase contracts (the “Purchase Contracts”) which may be issued under one or more purchase contract agreements (each, a “Purchase Contract Agreement”) to be entered between the Company and the purchase contract agent to be named therein (the “Purchase Contract Agent”); and (g) units (the “Units”) to be issued under one or more unit agreements to be entered into among the Company, a bank or trust company to be named therein, as unit agent (the “Unit Agent”), and the holders from time to time of the Units (each such unit agreement, a “Unit Agreement”).

We, as your counsel, 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 or advisable for the purposes of rendering this opinion.

On the basis of the foregoing, we advise you that, in our opinion:

1. When the necessary corporate action on the part of the Company has been taken to authorize the issuance and sale of such shares of Common Stock proposed to be sold by the Company, and when such shares of Common Stock are issued and delivered in accordance with the applicable underwriting or other agreement, such shares of Common Stock will be validly issued, fully paid and non-assessable.

2. Upon designation of the relative rights, preferences and limitations of any series of Preferred Stock by the Board of Directors of the Company and the proper filing with the

Secretary of State of the State of Delaware of a Certificate of Designation relating to such series of Preferred Stock, all necessary corporate action on the part of the Company will have been taken to authorize the issuance and sale of such series of Preferred Stock proposed to be sold by the Company, and when such shares of Preferred Stock are issued and delivered in accordance with the applicable underwriting or other agreement, such shares of Preferred Stock will be validly issued, fully paid and non-assessable.

3. When the Depositary Shares have been duly authorized, the Deposit Agreement relating to the Depositary Shares and the depositary receipts evidencing rights in the Depositary Shares have been duly executed and delivered by the Company and the relevant depositary and registered, and the Depositary Shares are sold and delivered at the price and in accordance with the terms set forth in the Registration Statement and any prospectus supplement relating thereto, the Deposit Agreement and the applicable underwriting or other agreement, the Depositary Shares will entitle the holders thereof to the rights specified in the Deposit Agreement and the depositary receipts evidencing rights therein, subject to applicable bankruptcy, insolvency and similar laws affecting creditors’ rights generally and equitable principles of general applicability.

4. When the Indentures and any supplemental indenture to be entered into in connection with the issuance of any Debt Securities have been duly authorized, executed and delivered by the Trustee and the Company; the specific terms of a particular series of Debt Securities have been duly authorized and established in accordance with the applicable Indenture; and such Debt Securities have been duly authorized, executed, authenticated, issued and delivered in accordance with such Indenture and the applicable underwriting or other agreement, such Debt Securities will constitute valid and binding obligations of the Company, enforceable in accordance with their terms, subject to applicable bankruptcy, insolvency and similar laws affecting creditors’ rights generally and equitable principles of general applicability.

5. When the Warrant Agreement to be entered into in connection with the issuance of any Warrants has been duly authorized, executed and delivered by the Warrant Agent and the Company; the specific terms of the Warrants have been duly authorized and established in accordance with the Warrant Agreement; and such Warrants and the warrant certificate have been duly authorized, executed, issued and delivered in accordance with the Warrant Agreement and the applicable underwriting or other agreement, such Warrants will constitute valid and binding obligations of the Company, enforceable in accordance with their terms, subject to applicable bankruptcy, insolvency and similar laws affecting creditors’ rights generally and equitable principles of general applicability.

6. When the Purchase Contract Agreement to be entered into in connection with the issuance of any Purchase Contracts has been duly authorized, executed and delivered by the Purchase Contract Agent and the Company; the specific terms of the Purchase Contracts have been duly authorized and established in accordance with the Purchase Contract Agreement; and such Purchase Contracts have been duly authorized, executed, issued and delivered in accordance with the Purchase Contract Agreement and the applicable underwriting or other agreement, such Purchase Contracts will constitute valid and binding obligations of the Company, enforceable in accordance with their terms, subject to applicable bankruptcy, insolvency and similar laws affecting creditors’ rights generally and equitable principles of general applicability.

7. When the Unit Agreement to be entered into in connection with the issuance of any Units has been duly authorized, executed and delivered by the Unit Agent and the Company; the specific terms of the Units have been duly authorized and established in accordance with the Unit Agreement; and such Units and the unit certificate have been duly

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authorized, executed, issued and delivered in accordance with the Unit Agreement and the applicable underwriting or other agreement, such Units will constitute valid and binding obligations of the Company, enforceable in accordance with their terms, subject to applicable bankruptcy, insolvency and similar laws affecting creditors’ rights generally and equitable principles of general applicability.

In connection with the opinions expressed above, we have assumed that, at or prior to the time of the delivery of any such security, (i) the Board of Directors of the Company shall have duly established the terms of such security and duly authorized the issuance and sale of such security and such authorization shall not have been modified or rescinded; (ii) the Registration Statement shall be effective and such effectiveness shall not have been terminated or rescinded; and (iii) there shall not have occurred any change in law affecting the validity or enforceability of such security. We have also assumed that none of the terms of any security to be established subsequent to the date hereof, nor the issuance and delivery of such security, nor the compliance by the Company with the terms of such security will violate any applicable law or will result in a violation of any provision of any instrument or agreement then binding upon the Company, or any restriction imposed by any court or governmental body having jurisdiction over the Company.

We are members of the Bar of the State of New York and the foregoing opinion is limited to the General Corporation Law of the State of Delaware.

We hereby consent to the filing of this opinion as an exhibit to the Registration Statement referred to above and further consent to the reference to our name under the caption “Legal Opinions” in the prospectus, which is a part of the Registration Statement. In giving this consent, we do not admit that we are in the category of persons whose consent is required under Section 7 of the Securities Act.

This opinion is rendered solely to you in connection with the above matter. This opinion may not be relied upon by you for any other purpose or relied upon by or furnished to any other person without our prior written consent.

Very truly yours,

/s/ Davis Polk & Wardwell

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