-----BEGIN PRIVACY-ENHANCED MESSAGE-----
Proc-Type: 2001,MIC-CLEAR
Originator-Name: webmaster@www.sec.gov
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 TWSM7vrzLADbmYQaionwg5sDW3P6oaM5D3tdezXMm7z1T+B+twIDAQAB
MIC-Info: RSA-MD5,RSA,
 MgRNpUrA/5x2W5WnaAOEpgDn2hZgnSgM+EvBBqqWgtgJjQbinEIjUAFUTd1uyiGL
 sZOPSKUsNDMXy+XnMmJ7LA==

<SEC-DOCUMENT>0001127602-11-006114.txt : 20110217
<SEC-HEADER>0001127602-11-006114.hdr.sgml : 20110217
<ACCEPTANCE-DATETIME>20110217155000
ACCESSION NUMBER:		0001127602-11-006114
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20110215
FILED AS OF DATE:		20110217
DATE AS OF CHANGE:		20110217

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			DUPERREAULT BRIAN
		CENTRAL INDEX KEY:			0001072522

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	001-05998
		FILM NUMBER:		11620902

	MAIL ADDRESS:	
		STREET 1:		C/O MARSH & MCLENNAN COMPANIES, INC.
		STREET 2:		1166 AVENUE OF THE AMERICAS
		CITY:			NEW YORK
		STATE:			NY
		ZIP:			10036

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
</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>2011-02-15</periodOfReport>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001072522</rptOwnerCik>
            <rptOwnerName>DUPERREAULT BRIAN</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>1166 AVENUE OF THE AMERICAS</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>NEW YORK</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10036</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>1</isOfficer>
            <officerTitle>President and CEO, MMC</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2011-02-15</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>M</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>64367</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>298075</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2011-02-15</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>F</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>32488</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>28.805</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>265587</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Restricted Stock Units</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F2"/>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2011-02-15</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>M</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>64367</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F3"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F3"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>64367</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>160857</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">Vesting and distribution to reporting person of 64,367 shares underlying restricted stock units of which 32,488 shares were withheld by Marsh &amp; McLennan Companies to cover applicable taxes.  These 64,367 shares underlying restricted stock units relate to 35,005 restricted stock units that were granted on February 23, 2009 and 29,362 restricted stock units that were granted on February 22, 2010.</footnote>
        <footnote id="F2">The security converts to Marsh &amp; McLennan Companies common stock on a 1-for-1 basis.</footnote>
        <footnote id="F3">Not Applicable.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Lucy Fato, Attorney-in-Fact</signatureName>
        <signatureDate>2011-02-17</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
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