<?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>0205 - Disclosure - Long-Term Debt</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>1/1/2011 - 3/31/2011
USD ($) / shares

USD ($)
</KeyName><CurrencySymbol>$</CurrencySymbol><contextRef><ContextID>Jan-01-2011_Mar-31-2011</ContextID><EntitySchema>http://www.sec.gov/CIK</EntitySchema><EntityValue>0001317630</EntityValue><PeriodDisplayName /><PeriodType>duration</PeriodType><PeriodStartDate>2011-01-01T00:00:00</PeriodStartDate><PeriodEndDate>2011-03-31T00:00:00</PeriodEndDate><Segments /><Scenarios /></contextRef><UPS><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><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></UPS><CurrencyCode>USD</CurrencyCode><OriginalCurrencyCode>USD</OriginalCurrencyCode></MCU><CurrencySymbol>$</CurrencySymbol><Labels><Label Id="1" Label="3 Months Ended" /><Label Id="2" Label="Mar. 31, 2011" /></Labels></Column></Columns><Rows><Row><Id>2</Id><IsAbstractGroupTitle>true</IsAbstractGroupTitle><Level>0</Level><ElementName>us-gaap_LongTermDebtAbstract</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>Long-Term Debt [Abstract]</Label></Row><Row><Id>3</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><Level>0</Level><ElementName>us-gaap_LongTermDebtTextBlock</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 5 - us-gaap:LongTermDebtTextBlock--&gt;
   &lt;div style="font-family: 'Times New Roman',Times,serif"&gt;
   &lt;div align="left" style="font-size: 10pt; margin-top: 12pt"&gt;&lt;b&gt;5. LONG-TERM DEBT&lt;/b&gt;
   &lt;/div&gt;
   &lt;div align="left" style="font-size: 10pt; margin-top: 6pt"&gt;&lt;b&gt;Derivative Instruments and Hedging Activities&lt;/b&gt;
   &lt;/div&gt;
   &lt;div align="left" style="font-size: 10pt; margin-top: 6pt"&gt;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;We use derivative financial instruments, including interest rate swap contracts, to manage our
   exposure to fluctuations in interest rates. The use of these financial instruments mitigates
   exposure to these risks and the variability of our operating results. We are not a party to
   leveraged derivatives and do not enter into derivative financial instruments for trading or
   speculative purposes. On September&amp;#160;27, 2010, ITC Holdings entered into a 10-year forward starting
   interest rate swap agreement (the &amp;#8220;September&amp;#160;2010 swap&amp;#8221;) with a notional amount of $50.0&amp;#160;million.
   Additionally, on March&amp;#160;16, 2011, ITC Holdings entered into two 10-year forward starting interest
   rate swap agreements (the &amp;#8220;March&amp;#160;2011 swaps&amp;#8221;) each with a notional amount of $25.0&amp;#160;million. The
   interest rate swaps manage interest rate risk associated with the forecasted future issuance of
   fixed-rate debt related to the expected refinancing of the maturing $267.0&amp;#160;million ITC Holdings
   5.25% Senior Notes due July&amp;#160;15, 2013.
   &lt;/div&gt;
   &lt;div align="left" style="font-size: 10pt; margin-top: 6pt"&gt;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;The interest rate swaps call for ITC Holdings to receive interest quarterly at a variable rate
   equal to LIBOR and to pay interest semi-annually at a fixed rate of 3.60% for the September&amp;#160;2010
   swap and a fixed rate of 4.45% for the March&amp;#160;2011 swaps effective for the ten-year period beginning
   July&amp;#160;15, 2013. The agreements will be terminated no later than the effective date of the interest
   rate swaps of July&amp;#160;15, 2013. The interest rate swaps have been determined to be highly effective at
   offsetting changes in the fair value of the forecasted interest cash flows associated with the
   expected debt issuance attributable to changes in benchmark interest rates from the trade date of
   the interest rate swaps to the issuance date of the debt obligation. As of March&amp;#160;31, 2011, there
   has been no ineffectiveness recorded on the condensed consolidated statement of operations. The
   interest rate swaps qualify for hedge accounting treatment, whereby any pre-tax gain or loss
   recognized from the trade date to the effective date for the effective portion of the hedge is
   recorded in accumulated other comprehensive income (loss). These amounts will be accumulated and
   amortized as a component of interest expense over the life of the forecasted debt. As of March&amp;#160;31,
   2011, the fair value of the derivative instruments was an asset of $4.3&amp;#160;million.
   &lt;/div&gt;
   &lt;div align="left" style="font-size: 10pt; margin-top: 12pt"&gt;&lt;b&gt;Revolving Credit Agreements&lt;/b&gt;
   &lt;/div&gt;
   &lt;div align="left" style="font-size: 10pt; margin-top: 6pt; margin-left: 1%"&gt;&lt;b&gt;&lt;i&gt;ITC Holdings Revolving Credit Agreement&lt;/i&gt;&lt;/b&gt;
   &lt;/div&gt;
   &lt;div align="left" style="font-size: 10pt; margin-top: 6pt"&gt;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;At March&amp;#160;31, 2011, ITC Holdings had no amounts outstanding under the ITC Holdings Revolving
   Credit Agreement.
   &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"&gt;
   &lt;div align="left" style="font-size: 10pt; margin-top: 6pt; margin-left: 1%"&gt;&lt;b&gt;&lt;i&gt;ITCTransmission/METC Revolving Credit Agreement&lt;/i&gt;&lt;/b&gt;
   &lt;/div&gt;
   &lt;div align="left" style="font-size: 10pt; margin-top: 6pt"&gt;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;At March&amp;#160;31, 2011, ITCTransmission and METC had $11.6&amp;#160;million and $12.8&amp;#160;million, respectively,
   outstanding under the ITCTransmission/METC Revolving Credit Agreement. The ITCTransmission/METC
   Revolving Credit Agreement has a scheduled maturity of March&amp;#160;29, 2012 and amounts outstanding as of
   March&amp;#160;31, 2011 are presented in the current liabilities section of our consolidated statement of
   financial position. The weighted-average interest rate on borrowings outstanding under the
   agreement was 0.5% at March&amp;#160;31, 2011.
   &lt;/div&gt;
   &lt;div align="left" style="font-size: 10pt; margin-top: 6pt; margin-left: 1%"&gt;&lt;b&gt;&lt;i&gt;ITC Midwest Revolving Credit Agreement&lt;/i&gt;&lt;/b&gt;
   &lt;/div&gt;
   &lt;div align="left" style="font-size: 10pt; margin-top: 6pt"&gt;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;On February&amp;#160;11, 2011, ITC Midwest entered into a new revolving credit agreement (the &amp;#8220;2011 ITC
   Midwest Revolving Credit Agreement&amp;#8221;) that establishes an unguaranteed, unsecured revolving facility
   under which ITC Midwest may borrow up to $75.0&amp;#160;million, in addition to the borrowing capacity under
   the ITC Midwest revolving credit agreement, dated as of January&amp;#160;28, 2008 (&amp;#8220;2008 ITC Midwest
   Revolving Credit Agreement&amp;#8221;). The maturity date of the 2011 ITC Midwest Revolving Credit Agreement
   is February&amp;#160;11, 2013. ITC Midwest&amp;#8217;s loans made under the 2011 ITC Midwest Revolving Credit
   Agreement bear interest at a variable rate, with rates on LIBOR-based loans varying from 125 to 150
   basis points over the applicable LIBOR rate, depending on ITC Midwest&amp;#8217;s credit rating, and rates on
   other loans at the higher of prime, 50 basis points over the federal funds rate or 100 basis points
   over the one month LIBOR plus an applicable margin varying from 25 basis points to 50 basis points
   in each case, depending on ITC Midwest&amp;#8217;s credit rating. The 2011 ITC Midwest Revolving Credit
   Agreement also provides for the payment to the lenders of a commitment fee on the average daily
   unused commitments at rates varying from 0.125% to 0.175% each year, depending on ITC Midwest&amp;#8217;s
   credit rating.
   &lt;/div&gt;
   &lt;div align="left" style="font-size: 10pt; margin-top: 6pt"&gt;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;At March&amp;#160;31, 2011, ITC Midwest had $49.6&amp;#160;million outstanding under the 2008 ITC Midwest
   Revolving Credit Agreement and the 2011 ITC Midwest Revolving Credit Agreement. The
   weighted-average interest rate on borrowings outstanding under the agreements was 0.8% at March&amp;#160;31,
   2011.
   &lt;/div&gt;
   &lt;div align="left" style="font-size: 10pt; margin-top: 6pt; margin-left: 1%"&gt;&lt;b&gt;&lt;i&gt;ITC Great Plains Revolving Credit Agreement&lt;/i&gt;&lt;/b&gt;
   &lt;/div&gt;
   &lt;div align="left" style="font-size: 10pt; margin-top: 6pt"&gt;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;On February&amp;#160;16, 2011, ITC Great Plains entered into a revolving credit agreement (the &amp;#8220;ITC
   Great Plains Revolving Credit Agreement&amp;#8221;) that established an unguaranteed, unsecured revolving
   credit facility under which ITC Great Plains may borrow and issue letters of credit up to $150.0
   million. The maturity date of the ITC Great Plains Revolving Credit Agreement is February&amp;#160;16, 2015.
   Loans made under the ITC Great Plains Revolving Credit Agreement will bear interest at a variable
   rate, with rates on LIBOR-based loans varying from 150 to 275 basis points over the applicable
   LIBOR rate, depending on ITC Great Plains&amp;#8217; credit rating and rates on other loans at the higher of
   prime, 50 basis points over the federal funds rate or 100 basis points over the one month LIBOR
   rate plus an applicable margin varying from 50 basis points to 175 basis points in each case,
   depending on ITC Great Plains&amp;#8217; credit rating. The ITC Great Plains Revolving Credit Agreement also
   provides for the payment to the lenders of a commitment fee on the average daily unused commitments
   at rates varying from 0.25% to 0.50% each year, depending on ITC Great Plains&amp;#8217; credit rating.
   &lt;/div&gt;
   &lt;div align="left" style="font-size: 10pt; margin-top: 6pt"&gt;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;At March&amp;#160;31, 2011, ITC Great Plains had $20.5&amp;#160;million outstanding under the ITC Great Plains
   Revolving Credit Agreement. The weighted-average interest rate on borrowings outstanding under the
   agreement was 2.0% at March&amp;#160;31, 2011.
   &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"&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>This element may be used as a single block of text to encapsulate the entire disclosure for long-term borrowings including data and tables.</ElementDefenition><ElementReferences>Reference 1: http://www.xbrl.org/2003/role/presentationRef
 -Publisher SEC
 -Name Regulation S-X (SX)
 -Number 210
 -Section 02
 -Paragraph 22
 -Article 5

</ElementReferences><IsTotalLabel>false</IsTotalLabel><IsEPS>false</IsEPS><Label>LONG-TERM DEBT</Label></Row></Rows><Footnotes /><NumberOfCols>1</NumberOfCols><NumberOfRows>2</NumberOfRows><ReportName>Long-Term Debt</ReportName><MonetaryRoundingLevel>UnKnown</MonetaryRoundingLevel><SharesRoundingLevel>UnKnown</SharesRoundingLevel><PerShareRoundingLevel>UnKnown</PerShareRoundingLevel><ExchangeRateRoundingLevel>UnKnown</ExchangeRateRoundingLevel><HasCustomUnits>false</HasCustomUnits><SharesShouldBeRounded>true</SharesShouldBeRounded></InstanceReport>
