﻿<?xml version="1.0" encoding="utf-8"?>
<InstanceReport xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <Version>2.2.0.7</Version>
  <hasSegments>false</hasSegments>
  <ReportName>Summary of Significant Accounting Policies</ReportName>
  <ReportLongName>0201 - Disclosure - Summary of Significant Accounting Policies</ReportLongName>
  <DisplayLabelColumn>true</DisplayLabelColumn>
  <ShowElementNames>false</ShowElementNames>
  <RoundingOption />
  <HasEmbeddedReports>false</HasEmbeddedReports>
  <Columns>
    <Column>
      <LabelColumn>false</LabelColumn>
      <Id>1</Id>
      <Labels>
        <Label Id="1" Label="6 Months Ended" />
        <Label Id="2" Label="Sep. 30, 2010" />
      </Labels>
      <CurrencyCode>USD</CurrencyCode>
      <FootnoteIndexer />
      <hasSegments>false</hasSegments>
      <hasScenarios>false</hasScenarios>
      <Segments />
      <Scenarios />
      <Units>
        <Unit>
          <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>
        </Unit>
        <Unit>
          <UnitID>Pure</UnitID>
          <UnitType>Standard</UnitType>
          <StandardMeasure>
            <MeasureSchema>http://www.xbrl.org/2003/instance</MeasureSchema>
            <MeasureValue>pure</MeasureValue>
            <MeasureNamespace>xbrli</MeasureNamespace>
          </StandardMeasure>
          <Scale>0</Scale>
        </Unit>
        <Unit>
          <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>
        </Unit>
        <Unit>
          <UnitID>Shares</UnitID>
          <UnitType>Standard</UnitType>
          <StandardMeasure>
            <MeasureSchema>http://www.xbrl.org/2003/instance</MeasureSchema>
            <MeasureValue>shares</MeasureValue>
            <MeasureNamespace>xbrli</MeasureNamespace>
          </StandardMeasure>
          <Scale>0</Scale>
        </Unit>
      </Units>
      <CurrencySymbol>$</CurrencySymbol>
    </Column>
  </Columns>
  <Rows>
    <Row>
      <Id>2</Id>
      <Label>Summary of Significant Accounting Policies [Abstract]</Label>
      <Level>0</Level>
      <ElementName>us-gaap_GeneralPoliciesAbstract</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>
      <IsAbstractGroupTitle>true</IsAbstractGroupTitle>
      <IsEquityPrevioslyReportedAsRow>false</IsEquityPrevioslyReportedAsRow>
      <IsEquityAdjustmentRow>false</IsEquityAdjustmentRow>
      <IsBeginningBalance>false</IsBeginningBalance>
      <IsEndingBalance>false</IsEndingBalance>
      <IsReverseSign>false</IsReverseSign>
      <PreferredLabelRole />
      <IsEPS>false</IsEPS>
      <FootnoteIndexer />
      <Cells>
        <Cell>
          <Id>1</Id>
          <ShowCurrencySymbol>false</ShowCurrencySymbol>
          <IsNumeric>false</IsNumeric>
          <IsRatio>false</IsRatio>
          <DisplayZeroAsNone>false</DisplayZeroAsNone>
          <NumericAmount>0</NumericAmount>
          <RoundedNumericAmount>0</RoundedNumericAmount>
          <NonNumbericText />
          <NonNumericTextHeader />
          <FootnoteIndexer />
          <hasSegments>false</hasSegments>
          <hasScenarios>false</hasScenarios>
          <DisplayDateInUSFormat>false</DisplayDateInUSFormat>
        </Cell>
      </Cells>
      <OriginalInstanceReportColumns />
      <ElementDataType>xbrli:stringItemType</ElementDataType>
      <SimpleDataType>string</SimpleDataType>
      <ElementDefenition>No definition available.</ElementDefenition>
      <IsTotalLabel>false</IsTotalLabel>
    </Row>
    <Row>
      <Id>3</Id>
      <Label>SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES</Label>
      <Level>1</Level>
      <ElementName>us-gaap_SignificantAccountingPoliciesTextBlock</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>
      <IsAbstractGroupTitle>false</IsAbstractGroupTitle>
      <IsEquityPrevioslyReportedAsRow>false</IsEquityPrevioslyReportedAsRow>
      <IsEquityAdjustmentRow>false</IsEquityAdjustmentRow>
      <IsBeginningBalance>false</IsBeginningBalance>
      <IsEndingBalance>false</IsEndingBalance>
      <IsReverseSign>false</IsReverseSign>
      <PreferredLabelRole>verboselabel</PreferredLabelRole>
      <IsEPS>false</IsEPS>
      <FootnoteIndexer />
      <Cells>
        <Cell>
          <Id>1</Id>
          <ShowCurrencySymbol>false</ShowCurrencySymbol>
          <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 1 - us-gaap:SignificantAccountingPoliciesTextBlock--&gt;
   &lt;div align="left" style="font-family: 'Times New Roman',Times,serif"&gt;
   &lt;!-- xbrl,ns --&gt;
   &lt;!-- xbrl,nx --&gt;
   &lt;div align="center" style="font-size: 10pt; margin-top: 0pt"&gt;&lt;b&gt;&lt;/b&gt;
   &lt;/div&gt;
   &lt;div align="left"&gt;
   &lt;/div&gt;
   &lt;div align="center" style="font-size: 10pt; margin-top: 0pt"&gt;&lt;b&gt;&lt;/b&gt;
   &lt;/div&gt;
   &lt;div align="left" style="font-size: 10pt; margin-top: 12pt"&gt;&lt;b&gt;1. SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES&lt;/b&gt;
   &lt;/div&gt;
   &lt;div align="left" style="font-size: 10pt; margin-top: 6pt"&gt;&lt;b&gt;&lt;i&gt;Basis of Presentation&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;Alkermes, Inc. (the &amp;#8220;Company&amp;#8221; or &amp;#8220;Alkermes&amp;#8221;) is a fully integrated biotechnology
   company committed to developing innovative medicines to improve patients&amp;#8217; lives. The
   Company developed, manufactures and commercializes VIVITROL&lt;sup style="font-size: 85%; vertical-align: text-top"&gt;&amp;#174;&lt;/sup&gt; for alcohol
   dependence and for the prevention of relapse to opioid dependence, following opioid
   detoxification. The Company also manufactures RISPERDAL&lt;sup style="font-size: 85%; vertical-align: text-top"&gt;&amp;#174;&lt;/sup&gt; CONSTA&lt;sup style="font-size: 85%; vertical-align: text-top"&gt;&amp;#174;&lt;/sup&gt; for
   schizophrenia and bipolar I disorder. The Company&amp;#8217;s pipeline includes extended-release
   injectable and oral products for the treatment of prevalent, chronic diseases, such as
   central nervous system (&amp;#8220;CNS&amp;#8221;) disorders, reward disorders, addiction, diabetes and
   autoimmune disorders. The Company is headquartered in Waltham, Massachusetts and has a
   research facility in Massachusetts and a commercial manufacturing facility in Ohio.
   &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 accompanying condensed consolidated financial statements of Alkermes for the
   three and six months ended September&amp;#160;30, 2010 and 2009 are unaudited and have been
   prepared on a basis substantially consistent with the audited financial statements for the
   year ended March&amp;#160;31, 2010. The year-end condensed consolidated balance sheet data was
   derived from audited financial statements, but does not include all disclosures required
   by accounting principles generally accepted in the United States of America (&amp;#8220;U.S.&amp;#8221;)
   (commonly referred to as &amp;#8220;GAAP&amp;#8221;). In the opinion of management, the condensed consolidated
   financial statements include all adjustments, which are of a normal recurring nature, that
   are necessary to present fairly the results of operations for the reported periods.
   &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;These financial statements should be read in conjunction with the Company&amp;#8217;s audited
   consolidated financial statements and notes thereto which are contained in the Company&amp;#8217;s
   Annual Report on Form 10-K for the year ended March&amp;#160;31, 2010, filed with the Securities
   and Exchange Commission (&amp;#8220;SEC&amp;#8221;). The results of the Company&amp;#8217;s operations for any interim
   period are not necessarily indicative of the results of the Company&amp;#8217;s operations for any
   other interim period or for a full fiscal year.
   &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;&lt;i&gt;Principles of Consolidation &lt;/i&gt;&amp;#8212; The condensed consolidated financial statements
   include the accounts of Alkermes, Inc. and its wholly-owned subsidiaries: Alkermes
   Controlled Therapeutics, Inc.; Alkermes Europe, Ltd.; and RC Royalty Sub LLC (&amp;#8220;Royalty
   Sub&amp;#8221;). Intercompany accounts and transactions have been eliminated.
   &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;&lt;i&gt;Use of Estimates &lt;/i&gt;&amp;#8212; The preparation of the Company&amp;#8217;s condensed consolidated financial
   statements in conformity with GAAP necessarily requires management to make estimates and
   assumptions that affect the following: (1)&amp;#160;reported amounts of assets and liabilities; (2)
   disclosure of contingent assets and liabilities at the date of the condensed consolidated
   financial statements; and (3)&amp;#160;the reported amounts of revenues and expenses during the
   reporting period. Actual results could differ from these estimates.
   &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;&lt;i&gt;Segment Information &lt;/i&gt;&amp;#8212; The Company operates as one business segment, which is the
   business of developing, manufacturing and commercializing innovative medicines designed to
   yield better therapeutic outcomes and improve the lives of patients with serious diseases.
   The Company&amp;#8217;s chief decision maker, the Chairman, President and Chief Executive Officer,
   reviews the Company&amp;#8217;s operating results on an aggregate basis and manages the Company&amp;#8217;s
   operations as a single operating unit.
   &lt;/div&gt;
   &lt;div align="left" style="font-size: 10pt; margin-top: 12pt"&gt;&lt;b&gt;&lt;i&gt;New Accounting Pronouncements&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;In September&amp;#160;2009, the Emerging Issues Task Force (&amp;#8220;EITF&amp;#8221;) of the Financial
   Accounting Standards Board (&amp;#8220;FASB&amp;#8221;) issued accounting guidance related to revenue
   recognition that amends the previous guidance on arrangements with multiple deliverables.
   The new guidance provides accounting principles and application guidance on whether
   multiple deliverables exist, how the arrangement should be separated, and provides for
   separate revenue recognition based upon management&amp;#8217;s estimate of the selling price for an
   undelivered item when there is no other means to determine the fair value of that
   undelivered item. Accounting guidance previously required that the fair value of the
   undelivered item be the price of the item either sold in a separate transaction between
   unrelated third parties or the price charged for each item when the item is sold
   separately by the vendor. This was difficult to determine when the product was not
   individually sold because of its unique features. Under the previous guidance, if the fair
   value of all of the elements in the arrangement was not determinable, then revenue was
   deferred until all of the items were delivered or fair value was determined. This guidance
   is effective prospectively for revenue arrangements entered into or materially modified in
   the Company&amp;#8217;s fiscal year beginning April&amp;#160;1, 2011, and the Company is currently evaluating
   the potential impact of this standard on its consolidated financial statements. Early
   adoption is permitted, however, adoption of this guidance as of a date other than April&amp;#160;1,
   2011 will require the Company to apply this guidance retrospectively effective as of April
   1, 2010, and will require disclosure of the effect of this guidance as applied to all
   previously reported interim periods in the fiscal year of adoption.
   &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="center" style="font-size: 10pt; margin-top: 0pt"&gt;
   &lt;b&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;In January&amp;#160;2010, the FASB issued accounting guidance related to fair value
   measurements that requires additional disclosure related to transfers in and out of Levels
   1 and 2 of the fair value hierarchy. The guidance also requires additional disclosure for
   activity within Level 3 of the fair value hierarchy. The guidance requires a reporting
   entity to disclose separately the amounts of significant transfers in and out of Level 1
   and Level 2 and describe the reasons for the transfers. In addition, this guidance
   requires a reporting entity to present information separately about purchases, sales
   issuances and settlements in the reconciliation for fair value measurements using
   significant unobservable inputs, or Level 3. This accounting standard was effective for
   interim and annual reporting periods beginning after December&amp;#160;31, 2009, other than for
   disclosures about purchases, sales, issuances and settlements in the roll forward of
   activity in Level 3 fair value measurements. Those disclosures are effective for fiscal
   years beginning after December&amp;#160;31, 2010 and for interim periods within those fiscal years.
   The Company adopted all provisions of this pronouncement, except for those related to the
   disclosure of disaggregated Level 3 activity, on January&amp;#160;1, 2010, and as this guidance
   only amends required disclosures in the Company&amp;#8217;s condensed consolidated financial
   statements, it did not have an effect upon the Company&amp;#8217;s financial position or results of
   operations. The Company does not expect the adoption of the remaining provisions of this
   amendment to have a significant impact on its consolidated financial statements.
   &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;In April&amp;#160;2010, the FASB issued accounting guidance related to the milestone method of
   revenue recognition for research and development arrangements. Under this guidance, the
   Company may recognize revenue contingent upon the achievement of a milestone in its
   entirety, in the period in which the milestone is achieved, only if the milestone meets
   all the criteria within the guidance to be considered substantive. This guidance is
   effective on a prospective basis for research and development milestones achieved in the
   Company&amp;#8217;s fiscal year beginning April&amp;#160;1, 2011. Early adoption is permitted, however,
   adoption of this guidance as of a date other than April&amp;#160;1, 2011 will require the Company
   to apply this guidance retrospectively effective as of April&amp;#160;1, 2010, and will require
   disclosure of the effect of this guidance as applied to all previously reported interim
   periods in the fiscal year of adoption. The Company plans to implement this guidance
   prospectively and the effect of this guidance will be limited to future transactions. The
   Company does not expect adoption of this standard to have a material impact on its
   financial position or results of operations.
   &lt;/div&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 />
          <hasSegments>false</hasSegments>
          <hasScenarios>false</hasScenarios>
          <DisplayDateInUSFormat>false</DisplayDateInUSFormat>
        </Cell>
      </Cells>
      <OriginalInstanceReportColumns />
      <ElementDataType>us-types:textBlockItemType</ElementDataType>
      <SimpleDataType>textblock</SimpleDataType>
      <ElementDefenition>This element may be used to describe all significant accounting policies of the reporting entity.</ElementDefenition>
      <ElementReferences>Reference 1: http://www.xbrl.org/2003/role/presentationRef
 -Publisher AICPA
 -Name Accounting Principles Board Opinion (APB)
 -Number 22
 -Paragraph 8

</ElementReferences>
      <IsTotalLabel>false</IsTotalLabel>
    </Row>
  </Rows>
  <Footnotes />
  <NumberOfCols>1</NumberOfCols>
  <NumberOfRows>2</NumberOfRows>
  <HasScenarios>false</HasScenarios>
  <MonetaryRoundingLevel>UnKnown</MonetaryRoundingLevel>
  <SharesRoundingLevel>UnKnown</SharesRoundingLevel>
  <PerShareRoundingLevel>UnKnown</PerShareRoundingLevel>
  <HasPureData>false</HasPureData>
  <SharesShouldBeRounded>true</SharesShouldBeRounded>
</InstanceReport>
