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Proc-Type: 2001,MIC-CLEAR
Originator-Name: webmaster@www.sec.gov
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 TWSM7vrzLADbmYQaionwg5sDW3P6oaM5D3tdezXMm7z1T+B+twIDAQAB
MIC-Info: RSA-MD5,RSA,
 DH/fjr6V7L8O8PiTqm/oXKnTpFX+qkJyP+l0GulNoJI8ohIM6uSUR+1hoD8mpFiD
 yy8eoCmD9tmkuItusN65LA==

<SEC-DOCUMENT>0001127602-10-016145.txt : 20100602
<SEC-HEADER>0001127602-10-016145.hdr.sgml : 20100602
<ACCEPTANCE-DATETIME>20100602184253
ACCESSION NUMBER:		0001127602-10-016145
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20100601
FILED AS OF DATE:		20100602
DATE AS OF CHANGE:		20100602

ISSUER:		

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			MARSH & MCLENNAN COMPANIES, INC.
		CENTRAL INDEX KEY:			0000062709
		STANDARD INDUSTRIAL CLASSIFICATION:	INSURANCE AGENTS BROKERS & SERVICES [6411]
		IRS NUMBER:				362668272
		STATE OF INCORPORATION:			DE
		FISCAL YEAR END:			1231

	BUSINESS ADDRESS:	
		STREET 1:		1166 AVENUE OF THE AMERICAS
		CITY:			NEW YORK
		STATE:			NY
		ZIP:			10036
		BUSINESS PHONE:		2123455000

	MAIL ADDRESS:	
		STREET 1:		1166 AVENUE OF THE AMERICAS
		CITY:			NEW YORK
		STATE:			NY
		ZIP:			10036

	FORMER COMPANY:	
		FORMER CONFORMED NAME:	MARSH & MCLENNAN COMPANIES INC
		DATE OF NAME CHANGE:	19920703

	FORMER COMPANY:	
		FORMER CONFORMED NAME:	MARLENNAN CORP
		DATE OF NAME CHANGE:	19760505

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			FANJUL OSCAR
		CENTRAL INDEX KEY:			0001194447

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	001-05998
		FILM NUMBER:		10874072

	MAIL ADDRESS:	
		STREET 1:		C/O OMEGA CAPITAL, S.L.
		STREET 2:		PASEO DE LA CASTELLANA, 28
		CITY:			MADRID
		STATE:			U3
		ZIP:			28046
</SEC-HEADER>
<DOCUMENT>
<TYPE>4
<SEQUENCE>1
<FILENAME>form4.xml
<DESCRIPTION>PRIMARY DOCUMENT
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0303</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2010-06-01</periodOfReport>

    <issuer>
        <issuerCik>0000062709</issuerCik>
        <issuerName>MARSH &amp; MCLENNAN COMPANIES, INC.</issuerName>
        <issuerTradingSymbol>MMC</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001194447</rptOwnerCik>
            <rptOwnerName>FANJUL OSCAR</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O OMEGA CAPITAL, S.L.</rptOwnerStreet1>
            <rptOwnerStreet2>PASEO DE LA CASTELLANA, 28</rptOwnerStreet2>
            <rptOwnerCity>MADRID</rptOwnerCity>
            <rptOwnerState></rptOwnerState>
            <rptOwnerZipCode>28046</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2010-06-01</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>4582.95</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>21.82</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>55653.144</value>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">Includes annual stock award of 4,582.95 shares pursuant to the MMC Directors' Stock Compensation Plan.</footnote>
        <footnote id="F2">Includes (i) a total of 1,487.8959 shares acquired pursuant to the Marsh &amp; McLennan dividend reinvestment plan from May 15, 2009 through May 15, 2010; (ii) 6,000 shares that were transferred from Diversity Sciav to Mr. Fanjul's direct holdings on May 27, 2010; and (iii) 3,700 shares that were transferred from an entity wholly owned by Mr. Fanjul and his spouse to Mr. Fanjul's direct holdings on May 27, 2010. Mr Fanjul and his spouse acquired 100% ownership of Diversity Sciav on May 11, 2010.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Lucy Fato, Attorney-in-Fact</signatureName>
        <signatureDate>2010-06-02</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
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