<?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>061200 - Disclosure - Customer Concentration</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>0001059562</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_RisksAndUncertaintiesAbstract</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>Customer Concentration [Abstract]</Label></Row><Row FlagID="0"><Id>2</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><LabelSeparator>

</LabelSeparator><Level>2</Level><ElementName>us-gaap_ConcentrationRiskDisclosureTextBlock</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="text-align: left; font-family: 'Times New Roman', serif; font-size: 10pt; font-weight: bold;"&gt;12.&amp;#160;&amp;#160;Customer Concentration&lt;/div&gt;&lt;div&gt;&lt;br /&gt;&lt;/div&gt;&lt;div style="text-align: left; font-family: 'Times New Roman', serif; font-size: 10pt;"&gt;Direct sales, through both of its business segments, to the Company's largest customer, Spirit Aerosystems ("Spirit"), accounted for 29.2% and 12.6% of the Company's total revenues for the three months ended June 30, 2013 and 2012, respectively.&amp;#160;&amp;#160;Direct sales to Spirit accounted for 29.0% and 12.5% of the Company's total revenues for the six months ended June 30, 2013 and 2012, respectively.&amp;#160;&amp;#160;Accounts receivable balances related to Spirit were 23.4% and 27.9% of the Company's total accounts receivable balance at June 30, 2013 and December 31, 2012, respectively.&lt;/div&gt;&lt;div&gt;&lt;br /&gt;&lt;/div&gt;&lt;div style="text-align: left; font-family: 'Times New Roman', serif; font-size: 10pt;"&gt;Direct sales, through both of its business segments, to the Company's second largest customer, Gulfstream Aerospace Corporation, a General Dynamics company ("Gulfstream"), accounted for 15.3% and 16.8% of the Company's total revenues for the three months ended June 30, 2013 and 2012, respectively.&amp;#160;&amp;#160;Direct sales to Gulfstream accounted for 16.8% and 16.3% of the Company's total revenues for the six months ended June 30, 2013 and 2012, respectively.&amp;#160;&amp;#160;Accounts receivable balances related to Gulfstream were 18.1% and 5.6% of the Company's total accounts receivable balance at June 30, 2013 and December 31, 2012, respectively.&lt;/div&gt;&lt;div&gt;&lt;br /&gt;&lt;/div&gt;&lt;div style="text-align: left; font-family: 'Times New Roman', serif; font-size: 10pt;"&gt;Direct sales, through both of its business segments, to the Company's third largest customer, The Boeing Company ("Boeing"), accounted for 15.0% and 20.1% of the Company's total revenues for the three months ended June 30, 2013 and 2012, respectively.&amp;#160;&amp;#160;Direct sales to Boeing accounted for 16.4% and 20.3% of the Company's total revenues for the six months ended June 30, 2013 and 2012, respectively.&amp;#160;&amp;#160;Accounts receivable balances based on direct sales related to Boeing were 9.0% and 11.3% of the Company's total accounts receivable balance at June 30, 2013 and December 31, 2012, respectively.&lt;/div&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 any concentrations existing at the date of the financial statements that make an entity vulnerable to a reasonably possible, near-term, severe impact. This disclosure informs financial statement users about the general nature of the risk associated with the concentration, and may indicate the percentage of concentration risk as of the balance sheet date.</ElementDefenition><ElementReferences>Reference 1: http://www.xbrl.org/2003/role/presentationRef

 -Publisher FASB

 -Name Statement of Financial Accounting Standard (FAS)

 -Number 107

 -Paragraph 15A

 -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.



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

 -Publisher FASB

 -Name Accounting Standards Codification

 -Topic 275

 -SubTopic 10

 -Section 50

 -Paragraph 20

 -URI http://asc.fasb.org/extlink&amp;oid=6927468&amp;loc=d3e6404-108592



Reference 3: http://www.xbrl.org/2003/role/presentationRef

 -Publisher FASB

 -Name Accounting Standards Codification

 -Topic 275

 -SubTopic 10

 -Section 50

 -Paragraph 16

 -URI http://asc.fasb.org/extlink&amp;oid=6927468&amp;loc=d3e6327-108592



Reference 4: http://www.xbrl.org/2003/role/presentationRef

 -Publisher AICPA

 -Name Statement of Position (SOP)

 -Number 94-6

 -Paragraph 21, 22, 24

 -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.



Reference 5: http://www.xbrl.org/2003/role/presentationRef

 -Publisher FASB

 -Name Accounting Standards Codification

 -Topic 825

 -SubTopic 10

 -Section 50

 -Paragraph 20

 -URI http://asc.fasb.org/extlink&amp;oid=7491637&amp;loc=d3e13531-108611



Reference 6: http://www.xbrl.org/2003/role/presentationRef

 -Publisher FASB

 -Name Accounting Standards Codification

 -Topic 275

 -SubTopic 10

 -Section 50

 -Paragraph 18

 -URI http://asc.fasb.org/extlink&amp;oid=6927468&amp;loc=d3e6351-108592



Reference 7: http://www.xbrl.org/2003/role/presentationRef

 -Publisher FASB

 -Name Accounting Standards Codification

 -Topic 825

 -SubTopic 10

 -Section 50

 -Paragraph 21

 -URI http://asc.fasb.org/extlink&amp;oid=7491637&amp;loc=d3e13537-108611



Reference 8: http://www.xbrl.org/2003/role/presentationRef

 -Publisher FASB

 -Name Accounting Standards Codification

 -Topic 275

 -SubTopic 10

 -Section 50

 -Paragraph 21

 -URI http://asc.fasb.org/extlink&amp;oid=6927468&amp;loc=d3e6442-108592



</ElementReferences><IsTotalLabel>false</IsTotalLabel><UnitID>0</UnitID><Label>Customer Concentration</Label></Row></Rows><Footnotes /><IsEquityReport>false</IsEquityReport><ReportName>Customer Concentration</ReportName><MonetaryRoundingLevel>UnKnown</MonetaryRoundingLevel><SharesRoundingLevel>UnKnown</SharesRoundingLevel><PerShareRoundingLevel>UnKnown</PerShareRoundingLevel><ExchangeRateRoundingLevel>UnKnown</ExchangeRateRoundingLevel><HasCustomUnits>true</HasCustomUnits><IsEmbedReport>false</IsEmbedReport><IsMultiCurrency>false</IsMultiCurrency><ReportType>Sheet</ReportType><RoleURI>http://lmiaerospace.com/role/CustomerConcentration</RoleURI><NumberOfCols>1</NumberOfCols><NumberOfRows>2</NumberOfRows></InstanceReport>
