﻿<?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>10701 - 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="6 Months Ended" />
        <Label Id="2" Label="Jun. 30, 2010" />
      </Labels>
      <CurrencyCode>USD</CurrencyCode>
      <FootnoteIndexer />
      <hasSegments>false</hasSegments>
      <hasScenarios>false</hasScenarios>
      <Segments />
      <Scenarios />
      <Units>
        <Unit>
          <UnitID>Unit12</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>Unit13</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>Unit1</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>5</Id>
      <Label>DEBT</Label>
      <Level>3</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 />
      <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;div&gt; &lt;div style="font-family: 'Times New Roman',Times,serif;"&gt;
&lt;div style="margin-top: 18pt; font-size: 10pt;" align="center"&gt;&lt;b&gt;NOTE 7&lt;br /&gt;DEBT&lt;/b&gt; &lt;/div&gt;
&lt;div style="margin-top: 12pt; font-size: 10pt;" align="left"&gt;&lt;i&gt;Convertible Debentures&lt;/i&gt; &lt;/div&gt;
&lt;div style="margin-top: 6pt; font-size: 10pt;" align="left"&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;On March&amp;nbsp;12, 2008, the Company issued and sold $181.5&amp;nbsp;million aggregate principal amount of senior convertible debentures due March&amp;nbsp;15, 2028 ("debentures"). The debentures pay interest at 1.875% per annum, payable semi-annually on March&amp;nbsp;15 and September&amp;nbsp;15 of each year, and commenced on September&amp;nbsp;15, 2008. The debentures will mature on March&amp;nbsp;15, 2028, subject to earlier repurchase or conversion. Each $1,000 principal amount of debentures is initially convertible, at the option of the holders, into approximately 42.5351 shares of the Company's common stock, at any time prior to the maturity date. The conversion rate is subject to certain adjustments, but will not be adjusted for accrued interest or any unpaid interest. The conversion rate initially represents a conversion price of $23.51 per share. Holders of the debentures may require the Company to repurchase all or a portion of their debentures on March&amp;nbsp;15, 2013, March 15, 2018 and March&amp;nbsp;15, 2023, or at any time before March&amp;nbsp;15, 2028 upon the occurrence of certain events including a change in control. The Company may redeem the debentures for cash beginning on or after March&amp;nbsp;22, 2013. &lt;/div&gt;
&lt;div style="margin-top: 6pt; font-size: 10pt;" align="left"&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;In October&amp;nbsp;2009, the Company undertook the exchange of $15&amp;nbsp;million face amount of the convertible debentures for 1.84&amp;nbsp;million shares of the Company's common stock. The debentures so acquired were retired. There is $166.5&amp;nbsp;million face value of the debentures outstanding as of June&amp;nbsp;30, 2010. &lt;/div&gt;
&lt;div style="margin-top: 6pt; font-size: 10pt;" align="left"&gt;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;Amortization expense related to the issuance costs of the debentures was $0.2&amp;nbsp;million and $0.3&amp;nbsp;million for the three- month periods ended June&amp;nbsp;30, 2010 and 2009, respectively, and $0.5 million for each of the six- month periods ended June&amp;nbsp;30, 2010 and 2009. The interest expense on the debentures was $0.8&amp;nbsp;million and $0.9&amp;nbsp;million, respectively, for the three- month periods ended June&amp;nbsp;30, 2010 and 2009, and $1.6&amp;nbsp;million and $1.7&amp;nbsp;million, respectively, for the six- month periods ended June&amp;nbsp;30, 2010 and 2009. The Company made cash payments of $1.6&amp;nbsp;million and $1.7&amp;nbsp;million for interest on the debentures during the six- month periods ended June&amp;nbsp;30, 2010 and 2009, respectively. &lt;/div&gt;&lt;/div&gt; &lt;/div&gt;</NonNumbericText>
          <NonNumericTextHeader>NOTE 7DEBT
Convertible Debentures
&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;&amp;nbsp;On March&amp;nbsp;12, 2008, the Company issued and sold $181.5&amp;nbsp;million aggregate principal</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>1</NumberOfRows>
  <HasScenarios>false</HasScenarios>
  <MonetaryRoundingLevel>UnKnown</MonetaryRoundingLevel>
  <SharesRoundingLevel>UnKnown</SharesRoundingLevel>
  <PerShareRoundingLevel>UnKnown</PerShareRoundingLevel>
  <HasPureData>false</HasPureData>
  <SharesShouldBeRounded>true</SharesShouldBeRounded>
</InstanceReport>
