<?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>2.2.0.7</Version>
  <hasSegments>false</hasSegments>
  <ReportName>Debt</ReportName>
  <ReportLongName>0209 - Disclosure - Debt</ReportLongName>
  <DisplayLabelColumn>true</DisplayLabelColumn>
  <ShowElementNames>false</ShowElementNames>
  <RoundingOption />
  <HasEmbeddedReports>false</HasEmbeddedReports>
  <Columns>
    <Column>
      <LabelColumn>false</LabelColumn>
      <Id>1</Id>
      <Labels>
        <Label Id="1" Label="9 Months Ended" />
        <Label Id="2" Label="Sep. 30, 2010" />
      </Labels>
      <CurrencyCode>USD</CurrencyCode>
      <FootnoteIndexer />
      <hasSegments>false</hasSegments>
      <hasScenarios>false</hasScenarios>
      <Segments />
      <Scenarios />
      <Units>
        <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>Pure</UnitID>
          <UnitType>Standard</UnitType>
          <StandardMeasure>
            <MeasureSchema>http://www.xbrl.org/2003/instance</MeasureSchema>
            <MeasureValue>pure</MeasureValue>
            <MeasureNamespace>xbrli</MeasureNamespace>
          </StandardMeasure>
          <Scale>0</Scale>
        </Unit>
        <Unit>
          <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>
        </Unit>
        <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>
      </Units>
      <CurrencySymbol>$</CurrencySymbol>
    </Column>
  </Columns>
  <Rows>
    <Row>
      <Id>2</Id>
      <Label>Debt [Abstract]</Label>
      <Level>0</Level>
      <ElementName>cop_DebtAbstract</ElementName>
      <ElementPrefix>cop</ElementPrefix>
      <IsBaseElement>false</IsBaseElement>
      <BalanceType>na</BalanceType>
      <PeriodType>duration</PeriodType>
      <ShortDefinition>Debt.</ShortDefinition>
      <IsReportTitle>false</IsReportTitle>
      <IsSegmentTitle>false</IsSegmentTitle>
      <IsSubReportEnd>false</IsSubReportEnd>
      <IsCalendarTitle>false</IsCalendarTitle>
      <IsTuple>false</IsTuple>
      <IsAbstractGroupTitle>true</IsAbstractGroupTitle>
      <IsEquityPrevioslyReportedAsRow>false</IsEquityPrevioslyReportedAsRow>
      <IsEquityAdjustmentRow>false</IsEquityAdjustmentRow>
      <IsBeginningBalance>false</IsBeginningBalance>
      <IsEndingBalance>false</IsEndingBalance>
      <IsReverseSign>false</IsReverseSign>
      <PreferredLabelRole />
      <IsEPS>false</IsEPS>
      <FootnoteIndexer />
      <Cells>
        <Cell>
          <Id>1</Id>
          <ShowCurrencySymbol>false</ShowCurrencySymbol>
          <IsNumeric>false</IsNumeric>
          <IsRatio>false</IsRatio>
          <DisplayZeroAsNone>false</DisplayZeroAsNone>
          <NumericAmount>0</NumericAmount>
          <RoundedNumericAmount>0</RoundedNumericAmount>
          <NonNumbericText />
          <NonNumericTextHeader />
          <FootnoteIndexer />
          <hasSegments>false</hasSegments>
          <hasScenarios>false</hasScenarios>
          <DisplayDateInUSFormat>false</DisplayDateInUSFormat>
        </Cell>
      </Cells>
      <OriginalInstanceReportColumns />
      <ElementDataType>xbrli:stringItemType</ElementDataType>
      <SimpleDataType>string</SimpleDataType>
      <ElementDefenition>Debt.</ElementDefenition>
      <IsTotalLabel>false</IsTotalLabel>
    </Row>
    <Row>
      <Id>3</Id>
      <Label>Debt</Label>
      <Level>1</Level>
      <ElementName>us-gaap_DebtDisclosureTextBlock</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>
      <IsAbstractGroupTitle>false</IsAbstractGroupTitle>
      <IsEquityPrevioslyReportedAsRow>false</IsEquityPrevioslyReportedAsRow>
      <IsEquityAdjustmentRow>false</IsEquityAdjustmentRow>
      <IsBeginningBalance>false</IsBeginningBalance>
      <IsEndingBalance>false</IsEndingBalance>
      <IsReverseSign>false</IsReverseSign>
      <PreferredLabelRole>verboselabel</PreferredLabelRole>
      <IsEPS>false</IsEPS>
      <FootnoteIndexer />
      <Cells>
        <Cell>
          <Id>1</Id>
          <ShowCurrencySymbol>false</ShowCurrencySymbol>
          <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 9 - us-gaap:DebtDisclosureTextBlock--&gt;
   &lt;div style="font-family: 'Times New Roman',Times,serif"&gt;
   &lt;div align="left" style="font-size: 10.5pt; margin-top: 10.5pt"&gt;&lt;b&gt;Note 9&amp;#8212;Debt&lt;/b&gt;
   &lt;/div&gt;
   &lt;div align="left" style="font-size: 10.5pt; margin-top: 10.5pt"&gt;We have two commercial paper programs supported by our $7.85&amp;#160;billion revolving credit facilities:
   the ConocoPhillips $6.35&amp;#160;billion program, primarily a funding source for short-term working capital
   needs, and the ConocoPhillips Qatar Funding Ltd. $1.5&amp;#160;billion commercial paper program, which is
   used to fund commitments relating to the Qatargas 3 Project. Commercial paper maturities are
   generally limited to 90&amp;#160;days.
   &lt;/div&gt;
   &lt;div align="left" style="font-size: 10.5pt; margin-top: 10.5pt"&gt;At both September&amp;#160;30, 2010, and December&amp;#160;31, 2009, we had no direct outstanding borrowings under
   our revolving credit facilities, but $40&amp;#160;million in letters of credit had been issued. In
   addition, under the two commercial paper programs, there was $1,159&amp;#160;million of commercial paper
   outstanding at September&amp;#160;30, 2010, compared with $1,300&amp;#160;million at December&amp;#160;31, 2009. Since we had
   $1,159&amp;#160;million of commercial paper outstanding and had issued $40&amp;#160;million of letters of credit, we
   had access to $6.7&amp;#160;billion in borrowing capacity under our revolving credit facilities at September
   30, 2010.
   &lt;/div&gt;
   &lt;div align="left" style="font-size: 10.5pt; margin-top: 10.5pt"&gt;During the first nine months of 2010, the following debt instruments were repaid prior to their
   maturity:
   &lt;/div&gt;
   &lt;div style="margin-top: 1pt"&gt;
   &lt;table width="100%" border="0" cellpadding="0" cellspacing="0" style="font-size: 10.5pt; text-align: left"&gt;
   &lt;tr valign="top" style="font-size: 10.5pt; color: #000000; background: transparent"&gt;
       &lt;td width="3%" style="background: transparent"&gt;&amp;#160;&lt;/td&gt;
       &lt;td width="3%" nowrap="nowrap" align="left"&gt;&lt;b&gt;&amp;#8226;&lt;/b&gt;&lt;/td&gt;
       &lt;td width="1%"&gt;&amp;#160;&lt;/td&gt;
       &lt;td&gt;The $400&amp;#160;million 6.68% bonds.&lt;/td&gt;
   &lt;/tr&gt;
   &lt;tr&gt;
       &lt;td style="font-size: 1pt"&gt;&amp;#160;&lt;/td&gt;
   &lt;/tr&gt;
   &lt;tr valign="top" style="font-size: 10.5pt; color: #000000; background: transparent"&gt;
       &lt;td width="3%" style="background: transparent"&gt;&amp;#160;&lt;/td&gt;
       &lt;td width="3%" nowrap="nowrap" align="left"&gt;&lt;b&gt;&amp;#8226;&lt;/b&gt;&lt;/td&gt;
       &lt;td width="1%"&gt;&amp;#160;&lt;/td&gt;
       &lt;td&gt;The $178&amp;#160;million 6.4% bonds.&lt;/td&gt;
   &lt;/tr&gt;
   &lt;tr&gt;
       &lt;td style="font-size: 1pt"&gt;&amp;#160;&lt;/td&gt;
   &lt;/tr&gt;
   &lt;tr valign="top" style="font-size: 10.5pt; color: #000000; background: transparent"&gt;
       &lt;td width="3%" style="background: transparent"&gt;&amp;#160;&lt;/td&gt;
       &lt;td width="3%" nowrap="nowrap" align="left"&gt;&lt;b&gt;&amp;#8226;&lt;/b&gt;&lt;/td&gt;
       &lt;td width="1%"&gt;&amp;#160;&lt;/td&gt;
       &lt;td&gt;The $1,750&amp;#160;million 6.35% bonds.&lt;/td&gt;
   &lt;/tr&gt;
   &lt;tr&gt;
       &lt;td style="font-size: 1pt"&gt;&amp;#160;&lt;/td&gt;
   &lt;/tr&gt;
   &lt;tr valign="top" style="font-size: 10.5pt; color: #000000; background: transparent"&gt;
       &lt;td width="3%" style="background: transparent"&gt;&amp;#160;&lt;/td&gt;
       &lt;td width="3%" nowrap="nowrap" align="left"&gt;&lt;b&gt;&amp;#8226;&lt;/b&gt;&lt;/td&gt;
       &lt;td width="1%"&gt;&amp;#160;&lt;/td&gt;
       &lt;td&gt;The $350&amp;#160;million 5.30% bonds.&lt;/td&gt;
   &lt;/tr&gt;
   &lt;tr&gt;
       &lt;td style="font-size: 1pt"&gt;&amp;#160;&lt;/td&gt;
   &lt;/tr&gt;
   &lt;tr valign="top" style="font-size: 10.5pt; color: #000000; background: transparent"&gt;
       &lt;td width="3%" style="background: transparent"&gt;&amp;#160;&lt;/td&gt;
       &lt;td width="3%" nowrap="nowrap" align="left"&gt;&lt;b&gt;&amp;#8226;&lt;/b&gt;&lt;/td&gt;
       &lt;td width="1%"&gt;&amp;#160;&lt;/td&gt;
       &lt;td&gt;The $750&amp;#160;million remaining balance of the Floating Rate Five-Year Term Notes.&lt;/td&gt;
   &lt;/tr&gt;
   &lt;/table&gt;
   &lt;/div&gt;
   &lt;div align="left" style="font-size: 10.5pt; margin-top: 10.5pt"&gt;During the first nine months of 2010, the following debt instruments were repaid at their maturity:
   &lt;/div&gt;
   &lt;div style="margin-top: 1pt"&gt;
   &lt;table width="100%" border="0" cellpadding="0" cellspacing="0" style="font-size: 10.5pt; text-align: left"&gt;
   &lt;tr valign="top" style="font-size: 10.5pt; color: #000000; background: transparent"&gt;
       &lt;td width="3%" style="background: transparent"&gt;&amp;#160;&lt;/td&gt;
       &lt;td width="3%" nowrap="nowrap" align="left"&gt;&lt;b&gt;&amp;#8226;&lt;/b&gt;&lt;/td&gt;
       &lt;td width="1%"&gt;&amp;#160;&lt;/td&gt;
       &lt;td&gt;The $150&amp;#160;million 9.875% bonds.&lt;/td&gt;
   &lt;/tr&gt;
   &lt;tr&gt;
       &lt;td style="font-size: 1pt"&gt;&amp;#160;&lt;/td&gt;
   &lt;/tr&gt;
   &lt;tr valign="top" style="font-size: 10.5pt; color: #000000; background: transparent"&gt;
       &lt;td width="3%" style="background: transparent"&gt;&amp;#160;&lt;/td&gt;
       &lt;td width="3%" nowrap="nowrap" align="left"&gt;&lt;b&gt;&amp;#8226;&lt;/b&gt;&lt;/td&gt;
       &lt;td width="1%"&gt;&amp;#160;&lt;/td&gt;
       &lt;td&gt;The $1,264&amp;#160;million 8.75% bonds.&lt;/td&gt;
   &lt;/tr&gt;
   &lt;/table&gt;
   &lt;/div&gt;
   &lt;div align="left" style="font-size: 10.5pt; margin-top: 10.5pt"&gt;At September&amp;#160;30, 2010, we classified $1,159&amp;#160;million of short-term debt as long-term debt, based on
   our ability and intent to refinance the obligation on a long-term basis under our revolving credit
   facilities.
   &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 />
          <hasSegments>false</hasSegments>
          <hasScenarios>false</hasScenarios>
          <DisplayDateInUSFormat>false</DisplayDateInUSFormat>
        </Cell>
      </Cells>
      <OriginalInstanceReportColumns />
      <ElementDataType>us-types:textBlockItemType</ElementDataType>
      <SimpleDataType>textblock</SimpleDataType>
      <ElementDefenition>Information about short-term and long-term debt arrangements, which includes amounts of borrowings under each line of credit, note payable, commercial paper issue, bonds indenture, debenture issue, and any other contractual agreement to repay funds, and about the underlying arrangements, rationale for a classification as long-term, including repayment terms, interest rates, collateral provided, restrictions on use of assets and activities, whether or not in compliance with debt covenants, and other matters important to users of the financial statements, such as the effects of refinancing and noncompliance with debt covenants.</ElementDefenition>
      <ElementReferences>Reference 1: http://www.xbrl.org/2003/role/presentationRef
 -Publisher SEC
 -Name Regulation S-X (SX)
 -Number 210
 -Section 02
 -Paragraph 19, 20, 22
 -Article 5

Reference 2: http://www.xbrl.org/2003/role/presentationRef
 -Publisher FASB
 -Name Statement of Financial Accounting Standard (FAS)
 -Number 129
 -Paragraph 2, 4

</ElementReferences>
      <IsTotalLabel>false</IsTotalLabel>
    </Row>
  </Rows>
  <Footnotes />
  <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>
