<?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>0021 - Disclosure - 15. Common Stock</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>From2011-07-01to2012-06-30</ContextID><EntitySchema>http://www.sec.gov/CIK</EntitySchema><EntityValue>0001020477</EntityValue><PeriodDisplayName /><PeriodType>duration</PeriodType><PeriodStartDate>2011-07-01T00:00:00</PeriodStartDate><PeriodEndDate>2012-06-30T00:00:00</PeriodEndDate><Segments /><Scenarios /></contextRef><UPS /><CurrencyCode /><OriginalCurrencyCode /></MCU><CurrencySymbol /><Labels><Label Key="CalendarSupplement" Id="0" Label="12 Months Ended" /><Label Key="Calendar" Id="1" Label="Jun. 30, 2012" /></Labels></Column></Columns><Rows><Row FlagID="0"><Id>1</Id><IsAbstractGroupTitle>true</IsAbstractGroupTitle><LabelSeparator>

</LabelSeparator><Level>1</Level><ElementName>ROBK_NotesToFinancialStatementsAbstract</ElementName><ElementPrefix>ROBK_</ElementPrefix><IsBaseElement>false</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>Notes to Financial Statements</Label></Row><Row FlagID="0"><Id>2</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><LabelSeparator>

</LabelSeparator><Level>2</Level><ElementName>ROBK_CommonStockTextBlock</ElementName><ElementPrefix>ROBK_</ElementPrefix><IsBaseElement>false</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="From2011-07-01to2012-06-30" UnitID=""><Id>1</Id><IsNumeric>false</IsNumeric><IsRatio>false</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>0</NumericAmount><RoundedNumericAmount>0</RoundedNumericAmount><NonNumbericText>&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;&lt;b&gt;Common and Preferred Shares&lt;/b&gt;&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0; text-indent: 0.5in"&gt;&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;On April 20, 2011, the Stockholders authorized an increase
the number of authorized common shares from 20,000,000 to 75,000,000, $0.001, par value, and authorized 5,000,000 preferred shares,
$.01, par value.&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;For the year ended June 30, 2011, the Company issued an aggregate
of 460,224 shares of common stock for legal services rendered, valued at $153,845, an average of $0.31, per share, the value of
the service provided.&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;For the year ended June 30, 2011, the Company issued an aggregate
of 779,526 shares of common stock for consulting services rendered under an extended contract, valued at $213,275, an average of
$0.24, per share, the value of the service provided.&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0; text-indent: 0.5in"&gt;&amp;#160;&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;For the year ended June 30, 2011, the Company issued 40,000
shares of common stock to board members for services rendered, valued at $9,600, an average of $0.24, per share, the value of the
service provided.&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;For the year ended June 30, 2011, the Company issued 1,600,000
shares of common stock as payment for loan payable - stockholder, valued at $320,000, $0.20, per share.&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;For the year ended June 30, 2011, the Company issued 139,270
shares of common stock as settlement of the derivative liability, valued at $52,923, $0.38, per share.&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;For the year ended June 30, 2011, the Company issued 50,000
shares of common stock in settlement of an account payable, valued at $11,012, $0.22, per share.&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;On June 10, 2011, in connection with the sale of the Series
A Preferred stock, the Company issued 20,000 shares of common stock, valued at $4,000, $0.20, per share, to the investment banker.&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0; text-indent: 0.5in"&gt;&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;In September 2011, the Company issued 7,900 shares of common
stock for the cashless exercise of&amp;#160;warrants.&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0; text-indent: 0.5in"&gt;&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;In September 2011, the Company issued an aggregate of 400,000
shares of common stock for payment of loan payable &amp;#150; stockholder, valued at $80,000, $0.20, per share.&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;In November 2011, the Company issued 65,058 shares of common
stock for the cashless exercise of warrants.&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;In December 2011, the Company issued an aggregate of 500,000
shares of common stock for payment of loan payable - stockholder, valued at $100,000, $0.20, per share.&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;In December 2011, the Company issued an aggregate of 5,847,089
shares of common stock as compensation to employees and consultants, valued at $1,169,417, $0.20, per share.&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0; text-indent: 0.5in"&gt;&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;In December 2011, the Company issued 300,000 shares of common
stock to board members for services rendered, valued at $60,000, $0.20, per share.&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;In February 2012, the Company issued an aggregate of 3,340,000
shares of common stock for payment of loan payable - stockholder, valued at $668,000, $0.20, per share.&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;In March 2012, the Company issued 200,000 shares of common
stock as repayment of accounts payable on behalf of RBMS, valued at $40,000, $0.20, per share.&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;In March 2012. The Company issued 80,000 shares of common
stock as payment for accounts payable valued at $20,000, $0.25, per share.&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;In April 2012, the Company issued 150,000 shares of common
stock as repayment of loans on behalf of RBMS, valued at $30,000, $0.20, per share&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;In April 2012, the Company issued 200,000 shares of common
stock for legal services rendered valued at $40,000, $0.20, per share.&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;&lt;b&gt;RBMS Equity&lt;/b&gt;&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;RBMS equity consists of 45 Series A Preferred Common Stock
Units and 2,687 Series B Preferred Common Stock Units. $1,925,000 Units were sold for cash from 2010 through 2012 and $550,000
Units were issued for services rendered to the Company.&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;Stock Option Plan&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0; text-indent: 0.5in"&gt;&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;On July 6, 2011, the Company&amp;#146;s stockholders approved
the Rotate Black, Inc. Stock Option Plan (Plan) under which the Chief Executive Officer of the Company may grant incentive stock
options to certain employees to purchase up to 25,000,000 shares of common stock of the Company. The option price shall be no less
than the fair market value of the stock, as defined. The Plan shall terminate after ten years.&amp;#160;&amp;#160;As June 30, 2012, no
options were granted under the Plan.&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;&lt;br /&gt;
&lt;b&gt;Warrants&lt;/b&gt;&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0; text-indent: 0.5in"&gt;&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;On May 28, 2010, the Company granted a warrant to purchase
100,000 shares of the Company's common stock to a consultant for services rendered. The warrant is exercisable at $0.17 and was
valued at $11,610 using a Black-Scholes Option Pricing Model with the stock price on day of grant, $0.19, the risk free interest
rate, 2.10% and the expected volatility of 69.81%. 7,900 shares of stock were issued pursuant to a cashless exercise of the warrant
on December 31, 2011.&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0; text-indent: 0.5in"&gt;&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;On June 23, 2011, in connection with the sale of Series A
Preferred Stock, the Company granted a five-year warrant to purchase 158,000 shares of the Company&amp;#146;s common stock to the
investment banker.&amp;#160;&amp;#160;The warrant was exercisable at $0.10, per share and was valued at $22,658 using a Black-Scholes Option
Pricing Model with&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;the stock price on day of grant, $0.19, per share, the risk
free interest rate of 1.48% and the expected volatility of 81.13%. 65,058 shares were subsequently issued pursuant to a cashless
exercise of the warrant on November 14, 2011.&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;&lt;b&gt;Class A 12% Preferred Stock&lt;/b&gt;&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;On June 10, 2011, the Board of Directors designated 500 shares
of Class A 12% Preferred stock (Series A), stated value of $1,000, per share. Each share is convertible at any time from and after
the issue date into shares of common stock determined by dividing the stated value of the shares of Series A by the conversion
price of $.10, as defined. Holders of the Series A are entitled to receive cumulative dividends at 12%, per annum, payable quarterly,
subject to periodic increases, as defined, and a late fee of 18%, per annum. The Series A have certain anti-dilution rights, as
defined. In addition, upon the occurrence of any triggering event, as defined, the holder of the Series A shall have the right
to: (A) require the Company to redeem all of the Series A held by the holder for a redemption price, in cash, equal to the an amount
as defined, or (B) redeem all of the Series A held by the holder for a redemption price, in shares of common stock of the Company,
equal to a number of shares equal to the redemption amount, as defined. Upon liquidation of the Company, the Series A holders are
entitled to receive an amount equal to the stated value, plus accrued and unpaid dividends. The Series A have no voting rights.&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0; text-indent: 0.5in"&gt;&amp;#160;&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;On June 10, 2011, the Company entered into a Securities Purchase
Agreement to sell up to an aggregate of 500 shares of Preferred Stock with an aggregate value of $500,000.&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0; text-indent: 0.5in"&gt;&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;As of June 30, 2011 the Company sold 190 Series A shares with
950,000 warrants to purchase common stock for an aggregate of $190,000. Each warrant is exercisable at $0.40, per share, for five
years. As of March 31, 2012, none of the warrants have been exercised.&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0; text-indent: 0.5in"&gt;&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;The fair value of the 950,000 detachable warrants sold with
the Series A for an aggregate of $190,000, was valued at $91,500 and recorded as additional paid-in capital using a Black Scholes
Option Pricing Model using the stock price on day of grant, $0.19, per share, the risk free interest rate of 1.48% and the expected
volatility of 81.13%.&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0; text-indent: 0.5in"&gt;&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;Since the Series A embodies an obligation to repurchase the
issuer&amp;#146;s equity shares in response to a triggering event, as defined, the Company has re-classified the Series A Preferred
Stock as a liability in accordance with guidance under ASC 480-10-65 and has recorded an expense of $91,500 to adjust the fair
value of the Series A Preferred Stock as a liability to $190,000.&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0; text-indent: 0.5in"&gt;&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;As of June 30, 2012, dividends on the Series A Preferred Stock
of $36,933 were accrued.&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0; text-indent: 0.5in"&gt;&amp;#160;&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;In connection with the sale of the Series A Preferred Stock,
the Company paid fees of $15,200, issued 20,000 shares of common stock, valued at $4,000, $0.20, per share, and granted warrants
to purchase 158,000 shares of common stock, valued at $22,658, to the investment banker.&lt;/p&gt;

&lt;p style="font: 8pt/normal Times New Roman, Times, Serif; margin: 0"&gt;&amp;#160;&lt;/p&gt;</NonNumbericText><FootnoteIndexer /><CurrencyCode /><CurrencySymbol /><IsIndependantCurrency>false</IsIndependantCurrency><ShowCurrencySymbol>false</ShowCurrencySymbol><DisplayDateInUSFormat>false</DisplayDateInUSFormat></Cell></Cells><ElementDataType>nonnum:textBlockItemType</ElementDataType><SimpleDataType>na</SimpleDataType><ElementDefenition>16. Common Stock</ElementDefenition><ElementReferences>No definition available.</ElementReferences><IsTotalLabel>false</IsTotalLabel><UnitID>0</UnitID><Label>15. Common Stock</Label></Row></Rows><Footnotes /><IsEquityReport>false</IsEquityReport><ReportName>15. Common Stock</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://rotateblack.com/role/CommonStock</RoleURI><NumberOfCols>1</NumberOfCols><NumberOfRows>2</NumberOfRows></InstanceReport>
