<?xml version="1.0" encoding="us-ascii"?><InstanceReport xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"><Version>2.2.0.25</Version><hasSegments>false</hasSegments><hasScenarios>false</hasScenarios><ReportLongName>0201 - Disclosure - Nature of Business</ReportLongName><DisplayLabelColumn>true</DisplayLabelColumn><ShowElementNames>false</ShowElementNames><RoundingOption /><HasEmbeddedReports>false</HasEmbeddedReports><Columns><Column><Id>1</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><LabelColumn>false</LabelColumn><CurrencyCode>USD</CurrencyCode><FootnoteIndexer /><hasSegments>false</hasSegments><hasScenarios>false</hasScenarios><MCU><KeyName>1/1/2010 - 12/31/2010
USD ($)

USD ($) / shares
</KeyName><CurrencySymbol>$</CurrencySymbol><contextRef><ContextID>Jan-01-2010_Dec-31-2010</ContextID><EntitySchema>http://www.sec.gov/CIK</EntitySchema><EntityValue>0001099590</EntityValue><PeriodDisplayName /><PeriodType>duration</PeriodType><PeriodStartDate>2010-01-01T00:00:00</PeriodStartDate><PeriodEndDate>2010-12-31T00:00:00</PeriodEndDate><Segments /><Scenarios /></contextRef><UPS><UnitProperty><UnitID>USD</UnitID><UnitType>Standard</UnitType><StandardMeasure><MeasureSchema>http://www.xbrl.org/2003/iso4217</MeasureSchema><MeasureValue>USD</MeasureValue><MeasureNamespace>iso4217</MeasureNamespace></StandardMeasure><Scale>0</Scale></UnitProperty><UnitProperty><UnitID>Shares</UnitID><UnitType>Standard</UnitType><StandardMeasure><MeasureSchema>http://www.xbrl.org/2003/instance</MeasureSchema><MeasureValue>shares</MeasureValue><MeasureNamespace>xbrli</MeasureNamespace></StandardMeasure><Scale>0</Scale></UnitProperty><UnitProperty><UnitID>USDEPS</UnitID><UnitType>Divide</UnitType><NumeratorMeasure><MeasureSchema>http://www.xbrl.org/2003/iso4217</MeasureSchema><MeasureValue>USD</MeasureValue><MeasureNamespace>iso4217</MeasureNamespace></NumeratorMeasure><DenominatorMeasure><MeasureSchema>http://www.xbrl.org/2003/instance</MeasureSchema><MeasureValue>shares</MeasureValue><MeasureNamespace>xbrli</MeasureNamespace></DenominatorMeasure><Scale>0</Scale></UnitProperty></UPS><CurrencyCode>USD</CurrencyCode><OriginalCurrencyCode>USD</OriginalCurrencyCode></MCU><CurrencySymbol>$</CurrencySymbol><Labels><Label Id="1" Label="12 Months Ended" /><Label Id="2" Label="Dec. 31, 2010" /></Labels></Column></Columns><Rows><Row><Id>2</Id><IsAbstractGroupTitle>true</IsAbstractGroupTitle><Level>0</Level><ElementName>us-gaap_GeneralPoliciesAbstract</ElementName><ElementPrefix>us-gaap</ElementPrefix><IsBaseElement>true</IsBaseElement><BalanceType>na</BalanceType><PeriodType>duration</PeriodType><ShortDefinition>No definition available.</ShortDefinition><IsReportTitle>false</IsReportTitle><IsSegmentTitle>false</IsSegmentTitle><IsSubReportEnd>false</IsSubReportEnd><IsCalendarTitle>false</IsCalendarTitle><IsTuple>false</IsTuple><IsEquityPrevioslyReportedAsRow>false</IsEquityPrevioslyReportedAsRow><IsEquityAdjustmentRow>false</IsEquityAdjustmentRow><IsBeginningBalance>false</IsBeginningBalance><IsEndingBalance>false</IsEndingBalance><IsReverseSign>false</IsReverseSign><PreferredLabelRole /><FootnoteIndexer /><Cells><Cell><Id>1</Id><IsNumeric>false</IsNumeric><IsRatio>false</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>0</NumericAmount><RoundedNumericAmount>0</RoundedNumericAmount><NonNumbericText /><NonNumericTextHeader /><FootnoteIndexer /><CurrencyCode /><CurrencySymbol /><IsIndependantCurrency>false</IsIndependantCurrency><ShowCurrencySymbol>false</ShowCurrencySymbol><DisplayDateInUSFormat>false</DisplayDateInUSFormat><hasSegments>false</hasSegments><hasScenarios>false</hasScenarios></Cell></Cells><OriginalInstanceReportColumns /><Unit>Other</Unit><ElementDataType>xbrli:stringItemType</ElementDataType><SimpleDataType>string</SimpleDataType><ElementDefenition>No definition available.</ElementDefenition><IsTotalLabel>false</IsTotalLabel><IsEPS>false</IsEPS><Label>Nature of Business [Abstract]</Label></Row><Row><Id>3</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><Level>0</Level><ElementName>us-gaap_NatureOfOperations</ElementName><ElementPrefix>us-gaap</ElementPrefix><IsBaseElement>true</IsBaseElement><BalanceType>na</BalanceType><PeriodType>duration</PeriodType><ShortDefinition>No definition available.</ShortDefinition><IsReportTitle>false</IsReportTitle><IsSegmentTitle>false</IsSegmentTitle><IsSubReportEnd>false</IsSubReportEnd><IsCalendarTitle>false</IsCalendarTitle><IsTuple>false</IsTuple><IsEquityPrevioslyReportedAsRow>false</IsEquityPrevioslyReportedAsRow><IsEquityAdjustmentRow>false</IsEquityAdjustmentRow><IsBeginningBalance>false</IsBeginningBalance><IsEndingBalance>false</IsEndingBalance><IsReverseSign>false</IsReverseSign><PreferredLabelRole>verboselabel</PreferredLabelRole><FootnoteIndexer /><Cells><Cell><Id>1</Id><IsNumeric>false</IsNumeric><IsRatio>false</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>0</NumericAmount><RoundedNumericAmount>0</RoundedNumericAmount><NonNumbericText>&lt;!--DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd" --&gt;
   &lt;!-- Begin Block Tagged Note 1 - us-gaap:NatureOfOperations--&gt;
   &lt;!-- xbrl,ns --&gt;
   &lt;!-- xbrl,nx --&gt;
   &lt;div style="font-family: 'Times New Roman',Times,serif; margin-left: 0in; "&gt;
   &lt;div align="left"&gt;
   &lt;/div&gt;
   &lt;div align="left" style="font-size: 10pt; margin-top: 0pt"&gt;&lt;b&gt;&lt;/b&gt;
   &lt;/div&gt;
   &lt;div align="left" style="font-size: 10pt; margin-top: 0pt"&gt;&lt;b&gt;&lt;/b&gt;
   &lt;/div&gt;
   &lt;div style="margin-top: 10pt"&gt;
   &lt;table width="100%" border="0" cellpadding="0" cellspacing="0" style="font-size: 10pt; text-align: left"&gt;
   &lt;tr valign="top" style="font-size: 10pt; color: #000000; background: transparent"&gt;
       &lt;td width="3%" nowrap="nowrap" align="left"&gt;&lt;b&gt;1.&lt;/b&gt;&lt;/td&gt;
       &lt;td width="1%"&gt;&amp;#160;&lt;/td&gt;
       &lt;td&gt;
   &lt;div style="text-align: justify"&gt;&lt;b&gt;Nature of Business&lt;/b&gt;
   &lt;/div&gt;&lt;/td&gt;
   &lt;/tr&gt;
   &lt;/table&gt;
   &lt;/div&gt;
   &lt;div align="justify" style="font-size: 10pt; margin-top: 10pt; margin-left: 4%"&gt;MercadoLibre Inc. (the &amp;#8220;Company&amp;#8221;) is an e-commerce enabler whose mission is to build the
   necessary online and technology tools to allow practically anyone to trade almost anything in
   Latin America.
   &lt;/div&gt;
   &lt;div align="justify" style="font-size: 10pt; margin-top: 10pt; margin-left: 4%"&gt;The Company pioneered online commerce in the region by developing a Web-based marketplace in
   which buyers and sellers are brought together to browse, buy and sell items such as computers,
   electronics, collectibles, automobiles, clothing and a host of practical and miscellaneous
   items. The Company&amp;#8217;s trading platform is a fully automated, topically arranged, online service.
   The Company&amp;#8217;s platform supports a fixed price format in which sellers and buyers trade items at
   a fixed price established by sellers, and an auction format in which sellers list items for
   sale and buyers bid on items of interest. Additionally, the Company introduced MercadoPago in
   2004, an integrated online payments solution. MercadoPago was designed to facilitate
   transactions on the MercadoLibre Marketplace by providing an escrow mechanism that enables
   users to send and receive payments online, and has experienced consistent growth since its
   launch.
   &lt;/div&gt;
   &lt;div align="justify" style="font-size: 10pt; margin-top: 10pt; margin-left: 4%"&gt;Since 2004, the Company introduced an online classifieds platform for motor vehicles,
   vessels and aircrafts and since 2006 the real state online classifieds platform. Buyers usually
   require a physical inspection of these items or specific types of interactions with the sellers
   before completing a transaction, and therefore an online classified advertisements service is
   better suited for purchase and sale of these types of items than the traditional online
   purchase and sale format. For these items, buyers can search by make, model, year and price,
   and sellers can list their phone numbers and receive prospective buyers&amp;#8217; e-mail addresses, in
   order to allow for instant and direct communication between sellers and potential buyers. For
   real estate listings, in addition to posting their contact information, individual owners or
   real estate agents can also upload pictures and videos of the property for sale and include
   maps of the property&amp;#8217;s location and layout. In addition, the Company launched several
   initiatives to improve its platform and expand its reach. Particularly relevant were the launch
   of eShops in 2006, a new platform tailored to attract lower rotation items and increase the
   breadth of products offered, the introduction of user generated information guides for buyers
   that improve the shopping experience, and the expansion of the online classifieds model by
   adding the services category. In terms of geographic expansion, the Company launched sites in
   Costa Rica, the Dominican Republic, and Panama.
   &lt;/div&gt;
   &lt;!-- Folio --&gt;
   &lt;!-- /Folio --&gt;
   &lt;/div&gt;
   &lt;!-- PAGEBREAK --&gt;
   &lt;div style="font-family: 'Times New Roman',Times,serif; margin-left: 0in; "&gt;
   &lt;div align="left" style="font-size: 10pt; margin-top: 0pt"&gt;
   &lt;b&gt;
   &lt;/b&gt;
   &lt;/div&gt;
   &lt;div style="margin-top: 0pt"&gt;
   &lt;table width="100%" border="0" cellpadding="0" cellspacing="0" style="font-size: 10pt"&gt;
   &lt;tr valign="top" style="font-size: 10pt; color: #000000; background: transparent"&gt;
   &lt;td width="3%" nowrap="nowrap" align="left"&gt;
   &lt;b&gt;
   &lt;/b&gt;
   &lt;/td&gt;
   &lt;td width="1%"&gt;&lt;/td&gt;
   &lt;td&gt;
   &lt;div style="text-align: justify"&gt;
   &lt;b&gt;
   &lt;/b&gt;
   &lt;/div&gt;
   &lt;/td&gt;
   &lt;/tr&gt;
   &lt;/table&gt;
   &lt;/div&gt;
   &lt;div align="justify" style="font-size: 10pt; margin-top: 10pt; margin-left: 4%"&gt;During 2007 the Company also launched a new and improved version of its MercadoPago payments
   platform in Chile and Colombia as well as in Argentina during 2008. The new MercadoPago,
   in addition to improving the ease of use and efficiency of payments for marketplace
   purchases, also allows for payments outside of the Company&amp;#8217;s marketplaces. Users are able
   to transfer money to other users with MercadoPago accounts and to incorporate MercadoPago
   as a means of payments in their independent commerce websites. In this way MercadoPago
   3.0 as it has been called is designed to meet the growing demand for Internet based
   payments systems in Latin America. In addition, in December&amp;#160;2009, the Company started
   processing off-MercadoLibre
   transactions
   with selected sites in Brazil as a Beta test using its new direct payments product, while maintaining the escrow product for
   on-MercadoLibre transactions. On March&amp;#160;30, 2010, the Company started processing
   off-MercadoLibre transactions through its new direct payments product to any site in Brazil
   which wants to adopt it. On July&amp;#160;16, 2010, the Company launched MercadoPago 3.0 in Brazil for
   all its marketplace transactions.
   &lt;/div&gt;
   &lt;div align="justify" style="font-size: 10pt; margin-top: 10pt; margin-left: 4%"&gt;As of December&amp;#160;31, 2010, the Company, through its wholly-owned subsidiaries, operated online
   commerce platforms directed towards Argentina, Brazil, Chile, Colombia, Costa Rica, Dominican
   Republic, Ecuador, Mexico, Panama, Peru, Portugal, Uruguay and Venezuela, and online payments
   solutions directed towards Argentina, Brazil, Mexico, Venezuela, Chile and Colombia. In
   addition, the Company operates a real estate classified platform that covers some areas of
   Florida, U.S.A.
   &lt;/div&gt;
   &lt;/div&gt;
</NonNumbericText><NonNumericTextHeader>&lt;!--DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd" --&gt;
   &lt;!-- Begin Block Tagged Note</NonNumericTextHeader><FootnoteIndexer /><CurrencyCode /><CurrencySymbol /><IsIndependantCurrency>false</IsIndependantCurrency><ShowCurrencySymbol>false</ShowCurrencySymbol><DisplayDateInUSFormat>false</DisplayDateInUSFormat><hasSegments>false</hasSegments><hasScenarios>false</hasScenarios></Cell></Cells><OriginalInstanceReportColumns /><Unit>Other</Unit><ElementDataType>us-types:textBlockItemType</ElementDataType><SimpleDataType>string</SimpleDataType><ElementDefenition>Describes the nature of an entity's business, the major products or services it sells or provides and its principal markets, including the locations of those markets. If the entity operates in more than one business, the disclosure also indicates the relative importance of its operations in each business and the basis for the determination (for example, assets, revenues, or earnings). Disclosures about the nature of operations need not be quantified; relative importance could be conveyed by use of terms such as "predominately", "about equally", or "major and other". This element is also referred to as "Business Description".</ElementDefenition><ElementReferences>Reference 1: http://www.xbrl.org/2003/role/presentationRef
 -Publisher AICPA
 -Name Statement of Position (SOP)
 -Number 94-6
 -Paragraph 10

</ElementReferences><IsTotalLabel>false</IsTotalLabel><IsEPS>false</IsEPS><Label>Nature of Business</Label></Row></Rows><Footnotes /><NumberOfCols>1</NumberOfCols><NumberOfRows>2</NumberOfRows><ReportName>Nature of Business</ReportName><MonetaryRoundingLevel>UnKnown</MonetaryRoundingLevel><SharesRoundingLevel>UnKnown</SharesRoundingLevel><PerShareRoundingLevel>UnKnown</PerShareRoundingLevel><ExchangeRateRoundingLevel>UnKnown</ExchangeRateRoundingLevel><HasCustomUnits>false</HasCustomUnits><SharesShouldBeRounded>true</SharesShouldBeRounded></InstanceReport>
