<?xml version="1.0" encoding="utf-8"?>
<InstanceReport xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <Version>1.0.0.3</Version>
  <hasSegments>false</hasSegments>
  <ReportName>Basis of Presentation</ReportName>
  <RoundingOption />
  <Columns>
    <Column>
      <LabelColumn>false</LabelColumn>
      <Id>1</Id>
      <Labels>
        <Label Id="1" Label="6 Months Ended" />
        <Label Id="2" Label="Jun. 30, 2009" />
        <Label Id="4" Label="USD / shares" />
      </Labels>
      <CurrencySymbol>$</CurrencySymbol>
      <hasSegments>false</hasSegments>
      <hasScenarios>false</hasScenarios>
      <Segments />
      <Scenarios />
      <Units>
        <Unit>
          <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>
        </Unit>
        <Unit>
          <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>
        </Unit>
        <Unit>
          <UnitID>usdPerShare</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>
        </Unit>
      </Units>
    </Column>
  </Columns>
  <Rows>
    <Row>
      <Id>2</Id>
      <Label>Notes to Financial Statements</Label>
      <Level>0</Level>
      <ElementName>ibm_NotesToFinancialStatementsAbstract</ElementName>
      <ElementPrefix>ibm</ElementPrefix>
      <IsBaseElement>false</IsBaseElement>
      <BalanceType>na</BalanceType>
      <PeriodType>duration</PeriodType>
      <ElementDataType>string</ElementDataType>
      <ShortDefinition>No definition available.</ShortDefinition>
      <IsReportTitle>false</IsReportTitle>
      <IsSegmentTitle>false</IsSegmentTitle>
      <IsSubReportEnd>false</IsSubReportEnd>
      <IsCalendarTitle>false</IsCalendarTitle>
      <IsTuple>false</IsTuple>
      <IsAbstractGroupTitle>true</IsAbstractGroupTitle>
      <IsBeginningBalance>false</IsBeginningBalance>
      <IsEndingBalance>false</IsEndingBalance>
      <IsEPS>false</IsEPS>
      <Cells>
        <Cell>
          <Id>1</Id>
          <ShowCurrencySymbol>false</ShowCurrencySymbol>
          <IsNumeric>false</IsNumeric>
          <NumericAmount>0</NumericAmount>
          <RoundedNumericAmount>0</RoundedNumericAmount>
          <NonNumbericText />
          <NonNumericTextHeader />
          <FootnoteIndexer />
          <hasSegments>false</hasSegments>
          <hasScenarios>false</hasScenarios>
        </Cell>
      </Cells>
      <ElementDefenition>No definition available.</ElementDefenition>
      <IsTotalLabel>false</IsTotalLabel>
    </Row>
    <Row>
      <Id>3</Id>
      <Label>Basis of Presentation:</Label>
      <Level>1</Level>
      <ElementName>us-gaap_OrganizationConsolidationAndPresentationOfFinancialStatementsDisclosureTextBlock</ElementName>
      <ElementPrefix>us-gaap</ElementPrefix>
      <IsBaseElement>true</IsBaseElement>
      <BalanceType>na</BalanceType>
      <PeriodType>duration</PeriodType>
      <ElementDataType>string</ElementDataType>
      <ShortDefinition>No definition available.</ShortDefinition>
      <IsReportTitle>false</IsReportTitle>
      <IsSegmentTitle>false</IsSegmentTitle>
      <IsSubReportEnd>false</IsSubReportEnd>
      <IsCalendarTitle>false</IsCalendarTitle>
      <IsTuple>false</IsTuple>
      <IsAbstractGroupTitle>false</IsAbstractGroupTitle>
      <IsBeginningBalance>false</IsBeginningBalance>
      <IsEndingBalance>false</IsEndingBalance>
      <IsEPS>false</IsEPS>
      <Cells>
        <Cell>
          <Id>1</Id>
          <ShowCurrencySymbol>false</ShowCurrencySymbol>
          <IsNumeric>false</IsNumeric>
          <NumericAmount>0</NumericAmount>
          <RoundedNumericAmount>0</RoundedNumericAmount>
          <NonNumbericText>&lt;!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"&gt;
&lt;HTML&gt;&lt;HEAD&gt;
&lt;META http-equiv=Content-Type content="text/html; charset=utf-8"&gt;
&lt;META content="MSHTML 6.00.2900.3492" name=GENERATOR&gt;&lt;/HEAD&gt;
&lt;BODY&gt;
&lt;P style="MARGIN: 0in 0in 0pt"&gt;&lt;FONT style="FONT-SIZE: 10pt"
face="Times New Roman" size=2&gt;&lt;/FONT&gt;&amp;nbsp;&lt;/P&gt;
&lt;P style="MARGIN: 0in 0in 0pt"&gt;&lt;B&gt;&lt;FONT
style="FONT-WEIGHT: bold; FONT-SIZE: 10pt" face="Times New Roman" size=2&gt;1.
&lt;U&gt;Basis of Presentation:&lt;/U&gt;&lt;/FONT&gt;&lt;/B&gt;&lt;FONT style="FONT-SIZE: 10pt"
size=2&gt;&amp;nbsp; The accompanying consolidated financial statements and footnotes
thereto are unaudited.&amp;nbsp; In the opinion of the management of the
International Business Machines Corporation (the company), these statements
include all adjustments, which are of a normal recurring nature, necessary to
present a fair statement of the company&amp;#146;s results of operations, financial
position and cash flows.&lt;/FONT&gt;&lt;/P&gt;
&lt;P style="MARGIN: 0in 0in 0pt"&gt;&lt;FONT style="FONT-SIZE: 10pt"
face="Times New Roman" size=2&gt;&lt;/FONT&gt;&amp;nbsp;&lt;/P&gt;
&lt;P style="MARGIN: 0in 0in 0pt; TEXT-INDENT: 0.25in"&gt;&lt;FONT
style="FONT-SIZE: 10pt" face="Times New Roman" size=2&gt;The preparation of
financial statements in conformity with accounting principles generally accepted
in the United States of America (GAAP) requires management to make estimates and
assumptions that affect the amounts of assets, liabilities, revenue, costs,
expenses and accumulated other comprehensive income/(loss) that are reported in
the Consolidated Financial Statements and accompanying disclosures. Actual
results may be different. See the company&amp;#146;s 2008 Annual Report for a discussion
of the company&amp;#146;s critical accounting estimates.&lt;/FONT&gt;&lt;/P&gt;
&lt;P style="MARGIN: 0in 0in 0pt"&gt;&lt;FONT style="FONT-SIZE: 10pt"
face="Times New Roman" size=2&gt;&lt;/FONT&gt;&amp;nbsp;&lt;/P&gt;
&lt;P style="MARGIN: 0in 0in 0pt; TEXT-INDENT: 0.25in"&gt;&lt;FONT
style="FONT-SIZE: 10pt" face="Times New Roman" size=2&gt;Interim results are not
necessarily indicative of results for a full year. The information included in
this Form&amp;nbsp;10-Q should be read in conjunction with the company&amp;#146;s 2008 Annual
Report.&lt;/FONT&gt;&lt;/P&gt;
&lt;P style="MARGIN: 0in 0in 0pt"&gt;&lt;FONT style="FONT-SIZE: 10pt"
face="Times New Roman" size=2&gt;&lt;/FONT&gt;&amp;nbsp;&lt;/P&gt;
&lt;P style="MARGIN: 0in 0in 0pt; TEXT-INDENT: 0.25in"&gt;&lt;FONT
style="FONT-SIZE: 10pt" face="Times New Roman" size=2&gt;The company evaluated
subsequent events through July&amp;nbsp;28, 2009, the date the company&amp;#146;s Board of
Directors reviewed the financial statements to be issued. The company issued the
financial statements on the same day.&lt;/FONT&gt;&lt;/P&gt;
&lt;P style="MARGIN: 0in 0in 0pt"&gt;&lt;FONT style="FONT-SIZE: 10pt"
face="Times New Roman" size=2&gt;&lt;/FONT&gt;&amp;nbsp;&lt;/P&gt;
&lt;P style="MARGIN: 0in 0in 0pt; TEXT-INDENT: 0.25in"&gt;&lt;FONT
style="FONT-SIZE: 10pt" face="Times New Roman" size=2&gt;Within the financial
tables in this Form&amp;nbsp;10-Q, certain columns and rows may not add due to the
use of rounded numbers for disclosure purposes. Percentages presented are
calculated from the underlying whole-dollar amounts.&lt;/FONT&gt;&lt;/P&gt;
&lt;P style="MARGIN: 0in 0in 0pt"&gt;&lt;FONT style="FONT-SIZE: 10pt"
face="Times New Roman" size=2&gt;&lt;/FONT&gt;&amp;nbsp;&lt;/P&gt;&lt;/BODY&gt;&lt;/HTML&gt;
</NonNumbericText>
          <NonNumericTextHeader>&amp;nbsp;
1.
Basis of Presentation:&amp;nbsp; The accompanying consolidated financial statements and footnotes
thereto are unaudited.&amp;nbsp; In the opinion of the</NonNumericTextHeader>
          <FootnoteIndexer />
          <hasSegments>false</hasSegments>
          <hasScenarios>false</hasScenarios>
        </Cell>
      </Cells>
      <ElementDefenition>No definition available.</ElementDefenition>
      <ElementReferences>No authoritative reference available.</ElementReferences>
      <IsTotalLabel>false</IsTotalLabel>
    </Row>
  </Rows>
  <Footnotes />
  <ComparabilityReport>false</ComparabilityReport>
  <NumberOfCols>1</NumberOfCols>
  <NumberOfRows>2</NumberOfRows>
  <HasScenarios>false</HasScenarios>
  <MonetaryRoundingLevel>UnKnown</MonetaryRoundingLevel>
  <SharesRoundingLevel>UnKnown</SharesRoundingLevel>
  <PerShareRoundingLevel>UnKnown</PerShareRoundingLevel>
  <HasPureData>false</HasPureData>
  <SharesShouldBeRounded>true</SharesShouldBeRounded>
</InstanceReport>
