<?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>0219 - Disclosure - Common Stock</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>1/1/2010 - 12/31/2010
USD ($)

USD ($) / shares
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   &lt;!-- Begin Block Tagged Note 19 - us-gaap:ScheduleOfStockByClassTextBlock--&gt;
   &lt;div style="margin-left: 0%"&gt;
   &lt;div style="margin-top: 12pt; font-size: 1pt"&gt;&amp;#160;
   &lt;/div&gt;
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       &lt;b&gt;&lt;font style="font-family: Arial, Helvetica"&gt;Note&amp;#160;19.&amp;#160;&lt;/font&gt;&lt;/b&gt;
   &lt;/td&gt;
       &lt;td&gt;
       &lt;b&gt;&lt;font style="font-family: Arial, Helvetica"&gt;Common
       Stock&lt;/font&gt;&lt;/b&gt;
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       During 2010, 2009 and 2008, 3&amp;#160;million, 1.6&amp;#160;million and
       1.2&amp;#160;million shares, respectively, of authorized but
       unissued shares of common stock were issued under the 2001
       Equity Compensation Plan and other employee share-based
       compensation plans.
   &lt;/div&gt;
   &lt;div style="margin-top: 6pt; font-size: 1pt"&gt;&amp;#160;
   &lt;/div&gt;
   &lt;div align="left" style="margin-left: 0%; margin-right: 0%; text-indent: 0%; font-size: 10pt; font-family: Arial, Helvetica; color: #000000; background: transparent"&gt;
       As of December&amp;#160;31, 2010, there were 9.9&amp;#160;million shares
       of common stock reserved for issuance under outstanding and
       future awards pursuant to the 2001 Equity Compensation Plan and
       other employee share-based compensation plans. During 2008, the
       Company registered 6.5&amp;#160;million shares of common stock
       reserved for issuance for future awards pursuant to the 2001
       Equity Compensation Plan and the Goodrich 2008 Global Employee
       Stock Purchase Plan.
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       The Company acquired 2.4&amp;#160;million, 0.4&amp;#160;million and
       2.6&amp;#160;million shares of treasury stock in 2010, 2009 and
       2008, respectively. Included in these amounts are shares the
       Company repurchased under its share repurchase program described
       below.
   &lt;/div&gt;
   &lt;div style="margin-top: 6pt; font-size: 1pt"&gt;&amp;#160;
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       A share repurchase program was initially approved by the
       Company&amp;#8217;s Board of Directors on October&amp;#160;24, 2006 and
       increased on February&amp;#160;19, 2008, for $600&amp;#160;million in
       total. On February&amp;#160;15,
   2011, the Board approved an additional increase to
       $1.1&amp;#160;billion in total. The primary purpose of the program
       is to reduce dilution to existing shareholders from the
       Company&amp;#8217;s share-based compensation plans. No time limit was
       set for completion of the program. Repurchases under the program
       may be made through open market or privately negotiated
       transactions at times and in such amounts as management deems
       appropriate, subject to market conditions, regulatory
       requirements and other factors. The program does not obligate
       the Company to repurchase any particular amount of common stock,
       and may be suspended or discontinued at any time without notice.
       The Company repurchased 2.2&amp;#160;million, 0.3&amp;#160;million and
       2.5&amp;#160;million shares of the Company&amp;#8217;s common stock for
       approximately $167&amp;#160;million, $16&amp;#160;million and
       $127&amp;#160;million in 2010, 2009 and 2008, respectively, under
       the program. The Company has $563.4&amp;#160;million remaining to
       repurchase shares under the program.
   &lt;/div&gt;
   &lt;/div&gt;
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 -Publisher SEC
 -Name Regulation S-X (SX)
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 -Paragraph 29
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
 -Publisher SEC
 -Name Regulation S-X (SX)
 -Number 210
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
 -Publisher FASB
 -Name Statement of Financial Accounting Standard (FAS)
 -Number 150
 -Paragraph 9

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