<?xml version="1.0" encoding="us-ascii"?><InstanceReport xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"><Version>2.4.0.8</Version><ReportLongName>060500 - Disclosure - Warranty</ReportLongName><DisplayLabelColumn>true</DisplayLabelColumn><ShowElementNames>false</ShowElementNames><RoundingOption /><HasEmbeddedReports>false</HasEmbeddedReports><Columns><Column FlagID="0"><Id>1</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><LabelSeparator>

</LabelSeparator><CurrencyCode /><FootnoteIndexer /><hasSegments>false</hasSegments><hasScenarios>false</hasScenarios><MCU><KeyName /><CurrencySymbol /><contextRef><ContextID>c20130101to20130630</ContextID><EntitySchema>http://www.sec.gov/CIK</EntitySchema><EntityValue>0001162461</EntityValue><PeriodDisplayName /><PeriodType>duration</PeriodType><PeriodStartDate>2013-01-01T00:00:00</PeriodStartDate><PeriodEndDate>2013-06-30T00:00:00</PeriodEndDate><Segments /><Scenarios /></contextRef><UPS /><CurrencyCode /><OriginalCurrencyCode /></MCU><CurrencySymbol /><Labels><Label Key="CalendarSupplement" Id="0" Label="6 Months Ended" /><Label Key="Calendar" Id="1" Label="Jun. 30, 2013" /></Labels></Column></Columns><Rows><Row FlagID="0"><Id>1</Id><IsAbstractGroupTitle>true</IsAbstractGroupTitle><LabelSeparator>

</LabelSeparator><Level>1</Level><ElementName>us-gaap_ProductWarrantiesDisclosuresAbstract</ElementName><ElementPrefix>us-gaap_</ElementPrefix><IsBaseElement>true</IsBaseElement><BalanceType>na</BalanceType><PeriodType>duration</PeriodType><IsReportTitle>false</IsReportTitle><IsSegmentTitle>false</IsSegmentTitle><IsCalendarTitle>false</IsCalendarTitle><IsEquityPrevioslyReportedAsRow>false</IsEquityPrevioslyReportedAsRow><IsEquityAdjustmentRow>false</IsEquityAdjustmentRow><IsBeginningBalance>false</IsBeginningBalance><IsEndingBalance>false</IsEndingBalance><IsReverseSign>false</IsReverseSign><FootnoteIndexer /><Cells><Cell FlagID="0" ContextID="" UnitID=""><Id>1</Id><IsNumeric>false</IsNumeric><IsRatio>false</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>0</NumericAmount><RoundedNumericAmount>0</RoundedNumericAmount><NonNumbericText /><FootnoteIndexer /><CurrencyCode /><CurrencySymbol /><IsIndependantCurrency>false</IsIndependantCurrency><ShowCurrencySymbol>false</ShowCurrencySymbol><DisplayDateInUSFormat>false</DisplayDateInUSFormat></Cell></Cells><ElementDataType>xbrli:stringItemType</ElementDataType><SimpleDataType>string</SimpleDataType><IsTotalLabel>false</IsTotalLabel><UnitID>0</UnitID><Label>Warranty [Abstract]</Label></Row><Row FlagID="0"><Id>2</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><LabelSeparator>

</LabelSeparator><Level>2</Level><ElementName>us-gaap_ProductWarrantyDisclosureTextBlock</ElementName><ElementPrefix>us-gaap_</ElementPrefix><IsBaseElement>true</IsBaseElement><BalanceType>na</BalanceType><PeriodType>duration</PeriodType><IsReportTitle>false</IsReportTitle><IsSegmentTitle>false</IsSegmentTitle><IsCalendarTitle>false</IsCalendarTitle><IsEquityPrevioslyReportedAsRow>false</IsEquityPrevioslyReportedAsRow><IsEquityAdjustmentRow>false</IsEquityAdjustmentRow><IsBeginningBalance>false</IsBeginningBalance><IsEndingBalance>false</IsEndingBalance><IsReverseSign>false</IsReverseSign><PreferredLabelRole>verboseLabel</PreferredLabelRole><FootnoteIndexer /><Cells><Cell FlagID="0" ContextID="c20130101to20130630" UnitID=""><Id>1</Id><IsNumeric>false</IsNumeric><IsRatio>false</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>0</NumericAmount><RoundedNumericAmount>0</RoundedNumericAmount><NonNumbericText>&lt;div style="font-family: 'Times New Roman', Times, serif; font-size: 10pt;"&gt;&lt;div style="font-size: 10pt; font-family: ''Times New Roman'', Times, serif; font-weight: bold; text-align: left;"&gt;Note 5. Warranty&lt;/div&gt;&lt;div&gt;&lt;br /&gt;&lt;/div&gt;&lt;div style="font-size: 10pt; font-family: ''Times New Roman'', Times, serif; text-align: left;"&gt;The Company provides a standard one-year warranty on all systems. Warranty coverage provided is for labor and parts necessary to repair the systems during the warranty period. The Company accounts for the estimated warranty cost of the standard warranty coverage as a charge to costs of revenue when revenue is recognized. The estimated warranty cost is based on historical product performance. To determine the estimated warranty reserve, the Company utilizes actual service records to calculate the average service expense per system and applies this to the equivalent number of units exposed under warranty. The Company updates these estimated charges every quarter.&lt;/div&gt;&lt;div&gt;&lt;br /&gt;&lt;/div&gt;&lt;div style="font-size: 10pt; font-family: ''Times New Roman'', Times, serif; text-align: left;"&gt;The following table provides the changes in the product warranty accrual for the six-month period ended June 30, 2013 &lt;font style="font-size: 10pt; font-family: ''Times New Roman'', Times, serif; font-style: italic;"&gt;(in thousands)&lt;/font&gt;:&lt;/div&gt;&lt;div&gt;&lt;br /&gt;&lt;/div&gt;&lt;table cellpadding="0" cellspacing="0" style="font-size: 10pt; font-family: 'Times New Roman', Times, serif; width: 100%;"&gt;&lt;tr&gt;&lt;td valign="bottom" style="vertical-align: bottom; padding-bottom: 2px;"&gt;&lt;div&gt;&amp;#160;&lt;/div&gt;&lt;/td&gt;&lt;td valign="bottom" style="vertical-align: bottom; padding-bottom: 2px;"&gt;&amp;#160;&lt;/td&gt;&lt;td colspan="2" valign="bottom" style="vertical-align: bottom; border-bottom: #000000 2px solid;"&gt;&lt;div style="font-size: 10pt; font-family: ''Times New Roman'', Times, serif; font-weight: bold; text-align: center;"&gt;Amount&lt;/div&gt;&lt;/td&gt;&lt;td nowrap="nowrap" valign="bottom" style="vertical-align: bottom; padding-bottom: 2px; text-align: left;"&gt;&amp;#160;&lt;/td&gt;&lt;/tr&gt;&lt;tr&gt;&lt;td valign="bottom" style="vertical-align: bottom; width: 88%; background-color: #cceeff;"&gt;&lt;div style="font-size: 10pt; font-family: ''Times New Roman'', Times, serif; text-align: left; margin-left: 16.2pt; text-indent: -16.2pt;"&gt;Beginning Balance &amp;#8211; December 31, 2012&lt;/div&gt;&lt;/td&gt;&lt;td valign="bottom" style="vertical-align: bottom; width: 1%; background-color: #cceeff;"&gt;&amp;#160;&lt;/td&gt;&lt;td valign="bottom" style="vertical-align: bottom; text-align: left; width: 1%; background-color: #cceeff;"&gt;&lt;div style="font-size: 10pt; font-family: ''Times New Roman'', Times, serif;"&gt;$&lt;/div&gt;&lt;/td&gt;&lt;td valign="bottom" style="vertical-align: bottom; text-align: right; width: 9%; background-color: #cceeff;"&gt;&lt;div style="font-size: 10pt; font-family: ''Times New Roman'', Times, serif;"&gt;1,212&lt;/div&gt;&lt;/td&gt;&lt;td nowrap="nowrap" valign="bottom" style="vertical-align: bottom; text-align: left; width: 1%; background-color: #cceeff;"&gt;&amp;#160;&lt;/td&gt;&lt;/tr&gt;&lt;tr&gt;&lt;td valign="bottom" style="vertical-align: bottom; width: 88%; background-color: #ffffff;"&gt;&lt;div style="font-size: 10pt; font-family: ''Times New Roman'', Times, serif; text-align: left; margin-left: 25.2pt; text-indent: -16.2pt;"&gt;Add: Accruals for warranties issued during the period&lt;/div&gt;&lt;/td&gt;&lt;td valign="bottom" style="vertical-align: bottom; width: 1%; background-color: #ffffff;"&gt;&amp;#160;&lt;/td&gt;&lt;td valign="bottom" style="vertical-align: bottom; text-align: left; width: 1%; background-color: #ffffff;"&gt;&amp;#160;&lt;/td&gt;&lt;td valign="bottom" style="vertical-align: bottom; text-align: right; width: 9%; background-color: #ffffff;"&gt;&lt;div style="font-size: 10pt; font-family: ''Times New Roman'', Times, serif;"&gt;1,712&lt;/div&gt;&lt;/td&gt;&lt;td nowrap="nowrap" valign="bottom" style="vertical-align: bottom; text-align: left; width: 1%; background-color: #ffffff;"&gt;&amp;#160;&lt;/td&gt;&lt;/tr&gt;&lt;tr&gt;&lt;td valign="bottom" style="vertical-align: bottom; padding-bottom: 2px; width: 88%; background-color: #cceeff;"&gt;&lt;div style="font-size: 10pt; font-family: ''Times New Roman'', Times, serif; text-align: left; margin-left: 25.2pt; text-indent: -16.2pt;"&gt;Less: Settlements made during the period&lt;/div&gt;&lt;/td&gt;&lt;td valign="bottom" style="vertical-align: bottom; padding-bottom: 2px; width: 1%; background-color: #cceeff;"&gt;&amp;#160;&lt;/td&gt;&lt;td valign="bottom" style="vertical-align: bottom; border-bottom: #000000 2px solid; text-align: left; width: 1%; background-color: #cceeff;"&gt;&amp;#160;&lt;/td&gt;&lt;td valign="bottom" style="vertical-align: bottom; border-bottom: #000000 2px solid; text-align: right; width: 9%; background-color: #cceeff;"&gt;&lt;div style="font-size: 10pt; font-family: ''Times New Roman'', Times, serif;"&gt;(1,875&lt;/div&gt;&lt;/td&gt;&lt;td nowrap="nowrap" valign="bottom" style="vertical-align: bottom; padding-bottom: 2px; text-align: left; width: 1%; background-color: #cceeff;"&gt;&lt;div style="font-size: 10pt; font-family: ''Times New Roman'', Times, serif;"&gt;)&lt;/div&gt;&lt;/td&gt;&lt;/tr&gt;&lt;tr&gt;&lt;td valign="bottom" style="vertical-align: bottom; padding-bottom: 4px; width: 88%; background-color: #ffffff;"&gt;&lt;div style="font-size: 10pt; font-family: ''Times New Roman'', Times, serif; text-align: left; margin-left: 16.2pt; text-indent: -16.2pt;"&gt;Ending Balance &amp;#8211;June 30, 2013&lt;/div&gt;&lt;/td&gt;&lt;td valign="bottom" style="vertical-align: bottom; padding-bottom: 4px; width: 1%; background-color: #ffffff;"&gt;&amp;#160;&lt;/td&gt;&lt;td valign="bottom" style="vertical-align: bottom; border-bottom: #000000 4px double; text-align: left; width: 1%; background-color: #ffffff;"&gt;&lt;div style="font-size: 10pt; font-family: ''Times New Roman'', Times, serif;"&gt;$&lt;/div&gt;&lt;/td&gt;&lt;td valign="bottom" style="vertical-align: bottom; border-bottom: #000000 4px double; text-align: right; width: 9%; background-color: #ffffff;"&gt;&lt;div style="font-size: 10pt; font-family: ''Times New Roman'', Times, serif;"&gt;1,049&lt;/div&gt;&lt;/td&gt;&lt;td nowrap="nowrap" valign="bottom" style="vertical-align: bottom; padding-bottom: 4px; text-align: left; width: 1%; background-color: #ffffff;"&gt;&amp;#160;&lt;/td&gt;&lt;/tr&gt;&lt;/table&gt;&lt;/div&gt;</NonNumbericText><FootnoteIndexer /><CurrencyCode /><CurrencySymbol /><IsIndependantCurrency>false</IsIndependantCurrency><ShowCurrencySymbol>false</ShowCurrencySymbol><DisplayDateInUSFormat>false</DisplayDateInUSFormat></Cell></Cells><ElementDataType>nonnum:textBlockItemType</ElementDataType><SimpleDataType>na</SimpleDataType><ElementDefenition>The entire disclosure for standard and extended product warranties and other product guarantee contracts, including a tabular reconciliation of the changes in the guarantor's aggregate product warranty liability for the reporting period.</ElementDefenition><ElementReferences>Reference 1: http://www.xbrl.org/2003/role/presentationRef

 -Publisher FASB

 -Name Accounting Standards Codification

 -Topic 460

 -SubTopic 10

 -Section 50

 -Paragraph 8

 -URI http://asc.fasb.org/extlink&amp;oid=6398077&amp;loc=d3e12565-110249



Reference 2: http://www.xbrl.org/2003/role/presentationRef

 -Publisher FASB

 -Name FASB Interpretation (FIN)

 -Number 45

 -Paragraph 14

 -Subparagraph a, b

 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.  This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.



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