<?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>107 - Disclosure - Summary of Significant Accounting Policies</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>P01_01_2013To06_30_2013</ContextID><EntitySchema>http://www.sec.gov/CIK</EntitySchema><EntityValue>0001230355</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_AccountingPoliciesAbstract</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>Accounting Policies [Abstract]</Label></Row><Row FlagID="0"><Id>2</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><LabelSeparator>

</LabelSeparator><Level>2</Level><ElementName>us-gaap_SignificantAccountingPoliciesTextBlock</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>label</PreferredLabelRole><FootnoteIndexer /><Cells><Cell FlagID="0" ContextID="P01_01_2013To06_30_2013" UnitID=""><Id>1</Id><IsNumeric>false</IsNumeric><IsRatio>false</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>0</NumericAmount><RoundedNumericAmount>0</RoundedNumericAmount><NonNumbericText>              &lt;table border="0" style="clear:both;width:100%; table-layout:fixed;"&gt;  &lt;tr&gt;  &lt;td&gt;&lt;/td&gt;  &lt;/tr&gt;  &lt;/table&gt;    &lt;div style="clear:both; FONT-FAMILY:Times New Roman;FONT-SIZE: 10pt;MARGIN: 0pt 0px; FONT: 10pt Times New Roman, Times, Serif "&gt;  &lt;div style="clear:both; FONT-FAMILY:Times New Roman;FONT-SIZE: 10pt;MARGIN: 0in 0in 0pt; FONT-SIZE: 10pt"&gt;  &lt;table style="clear:both;MARGIN-TOP: 0pt; WIDTH: 100%; FONT: 10pt Times New Roman, Times, Serif; MARGIN-BOTTOM: 0pt"   cellspacing="0" cellpadding="0"&gt;  &lt;tr style="VERTICAL-ALIGN: top"&gt;  &lt;td style="WIDTH: 0.25in"&gt;  &lt;div style="clear:both; FONT-FAMILY:Times New Roman;FONT-SIZE: 10pt"&gt;&lt;font  style="FONT-FAMILY: Times New Roman, Times, Serif; COLOR: windowtext"&gt;&lt;b&gt;  2.&lt;/b&gt;&lt;/font&gt;&lt;/div&gt;  &lt;/td&gt;  &lt;td&gt;  &lt;div style="clear:both; FONT-FAMILY:Times New Roman;FONT-SIZE: 10pt"&gt;&lt;b&gt;  Summary of Significant Accounting Policies&amp;#160;&lt;/b&gt;&lt;/div&gt;  &lt;/td&gt;  &lt;/tr&gt;  &lt;/table&gt;  &lt;/div&gt;    &lt;div style="clear:both; FONT-FAMILY:Times New Roman;FONT-SIZE: 10pt;MARGIN: 0pt 0px; FONT: 10pt Times New Roman, Times, Serif"&gt;  &lt;b&gt;&lt;i&gt;&amp;#160;&lt;/i&gt;&lt;/b&gt;&lt;/div&gt;    &lt;div style="clear:both; FONT-FAMILY:Times New Roman;FONT-SIZE: 10pt;MARGIN: 0pt 0px; FONT: 10pt Times New Roman, Times, Serif"&gt;  &lt;b&gt;&lt;i&gt;Basis of presentation&lt;/i&gt;&lt;/b&gt;&lt;/div&gt;    &lt;div style="clear:both; FONT-FAMILY:Times New Roman;FONT-SIZE: 10pt;MARGIN: 0pt 0px; FONT: 10pt Times New Roman, Times, Serif"&gt;  &amp;#160;&lt;/div&gt;    &lt;div style="clear:both; FONT-FAMILY:Times New Roman;FONT-SIZE: 10pt;TEXT-ALIGN: justify; MARGIN: 0pt 0px; FONT: 10pt Times New Roman, Times, Serif"&gt;  The Company has prepared the accompanying consolidated financial  statements in conformity with accounting principles generally  accepted in the United States of America (&amp;#8220;U.S. GAAP&amp;#8221;)  for interim financial information and pursuant to the rules and  regulations of the Securities and Exchange Commission. The  consolidated financial statements are unaudited and should be read  in conjunction with the audited consolidated financial statements  included in the Company's Annual Report on Form 10-K for the year  ended December 31, 2012. The accompanying unaudited interim  consolidated financial statements reflect all adjustments  (consisting of normal recurring adjustments) which are, in the  opinion of the Company&amp;#8217;s management, necessary for a fair  statement of the Company&amp;#8217;s consolidated financial position,  results of operations and cash flows for the periods presented.  These principles require management to make estimates and  assumptions that affect the reported amounts of assets and  liabilities and disclosure of contingent assets and liabilities at  the date of the consolidated financial statements and the reported  amounts of revenues and expenses during the reporting period. The  principal estimates relate to accounts receivable reserves,  inventory valuation, stock-based compensation, accrued expenses and  income tax valuation. Actual results could differ from those  estimates. The year-end balance sheet data was derived from audited  financial statements, but does not include all disclosures required  by U.S. GAAP. All intercompany accounts and transactions have been  eliminated in consolidation.&lt;/div&gt;    &lt;div style="clear:both; FONT-FAMILY:Times New Roman;FONT-SIZE: 10pt"&gt;  &amp;#160;&lt;/div&gt;    &lt;div style="clear:both; FONT-FAMILY:Times New Roman;FONT-SIZE: 10pt;TEXT-ALIGN: justify; MARGIN: 0pt 0px; FONT: 10pt Times New Roman, Times, Serif"&gt;  The accompanying financial statements have been prepared on a going  concern basis, which contemplates the realization of assets and the  satisfaction of liabilities and commitments in the normal course of  business. The Company has incurred operating losses and negative  cash flows from operations, which raise substantial doubt about its  ability to continue as a going concern. The financial statements do  not include any adjustments relating to the recoverability and  classification of recorded asset amounts or amounts of liabilities  that might result from the outcome of this uncertainty. To meet its  capital needs, the Company is considering multiple alternatives,  including, but not limited to, additional equity financings, debt  financings and other funding transactions. There can be no  assurance that the Company will be able to complete any such  transaction on acceptable terms or otherwise. If the Company is  unable to obtain the necessary capital, it will need to pursue a  plan to license or sell its assets, seek to be acquired by another  entity, cease operations and/or seek bankruptcy protection.&lt;/div&gt;    &lt;div style="clear:both; FONT-FAMILY:Times New Roman;FONT-SIZE: 10pt;TEXT-ALIGN: justify; MARGIN: 0pt 0px; FONT: 10pt Times New Roman, Times, Serif"&gt;  &amp;#160;&lt;/div&gt;    &lt;div style="clear:both; FONT-FAMILY:Times New Roman;FONT-SIZE: 10pt;TEXT-ALIGN: justify; MARGIN: 0pt 0px; FONT: 10pt Times New Roman, Times, Serif"&gt;  For a description of our critical accounting policies and  estimates, please refer to the &amp;#8220;Critical Accounting Policies  and Estimates&amp;#8221; section of the &amp;#8220;Management&amp;#8217;s  Discussion and Analysis of Financial Condition and Results of  Operations&amp;#8221; section contained in our Annual Report on Form  10-K for the year ended December 31, 2012. There have been no  material changes in any of our accounting policies since December  31, 2012, however&amp;#160;the Company has&amp;#160;adopted the following  new accounting policies related to the Merger.&lt;/div&gt;    &lt;div style="clear:both; FONT-FAMILY:Times New Roman;FONT-SIZE: 10pt;TEXT-ALIGN: justify; MARGIN: 0pt 0px; FONT: 10pt Times New Roman, Times, Serif"&gt;  &amp;#160;&lt;/div&gt;    &lt;div style="clear:both; FONT-FAMILY:Times New Roman;FONT-SIZE: 10pt;TEXT-ALIGN: justify; MARGIN: 0pt 0px; FONT: 10pt Times New Roman, Times, Serif"&gt;  &lt;b&gt;&lt;i&gt;Identifiable Intangible Assets and Goodwill&lt;/i&gt;&lt;/b&gt;&lt;/div&gt;    &lt;div style="clear:both; FONT-FAMILY:Times New Roman;FONT-SIZE: 10pt;TEXT-ALIGN: justify; MARGIN: 0pt 0px; FONT: 10pt Times New Roman, Times, Serif"&gt;  &amp;#160;&lt;/div&gt;    &lt;div style="clear:both; FONT-FAMILY:Times New Roman;FONT-SIZE: 10pt;TEXT-ALIGN: justify; MARGIN: 0pt 0px; FONT: 10pt Times New Roman, Times, Serif"&gt;  Identifiable intangible assets are recorded at cost, or when  acquired as part of a business acquisition, at estimated fair  value. Certain intangible assets are amortized over periods ranging  from 10 to 17 years. Similar to tangible personal property, plant  and equipment,&amp;#160;the Company&amp;#160;periodically evaluates  identifiable intangible assets for impairment whenever events or  changes in circumstances indicate that the carry amount may not be  recoverable.&lt;/div&gt;    &lt;div style="clear:both; FONT-FAMILY:Times New Roman;FONT-SIZE: 10pt;TEXT-ALIGN: justify; MARGIN: 0pt 0px; FONT: 10pt Times New Roman, Times, Serif"&gt;  &amp;#160;&lt;/div&gt;    &lt;div style="clear:both; FONT-FAMILY:Times New Roman;FONT-SIZE: 10pt;TEXT-ALIGN: justify; MARGIN: 0pt 0px; FONT: 10pt Times New Roman, Times, Serif"&gt;  Indefinite-lived intangible assets, such as goodwill and certain  trademarks, are not amortized. The Company tests the carrying  amounts of these assets for recoverability on an annual basis or  when events or changes in circumstances indicate evidence of  potential impairment exists, using a fair value based test.&lt;/div&gt;    &lt;div style="clear:both; FONT-FAMILY:Times New Roman;FONT-SIZE: 10pt;TEXT-ALIGN: justify; MARGIN: 0pt 0px; FONT: 10pt Times New Roman, Times, Serif"&gt;  &amp;#160;&lt;/div&gt;    &lt;div style="clear:both; FONT-FAMILY:Times New Roman;FONT-SIZE: 10pt;TEXT-ALIGN: justify; MARGIN: 0pt 0px; FONT: 10pt Times New Roman, Times, Serif"&gt;  &lt;b&gt;&lt;i&gt;Debt Issuance Costs&lt;/i&gt;&lt;/b&gt;&lt;/div&gt;    &lt;div style="clear:both; FONT-FAMILY:Times New Roman;FONT-SIZE: 10pt;TEXT-ALIGN: justify; MARGIN: 0pt 0px; FONT: 10pt Times New Roman, Times, Serif"&gt;  &amp;#160;&lt;/div&gt;    &lt;div style="clear:both; FONT-FAMILY:Times New Roman;FONT-SIZE: 10pt;TEXT-ALIGN: justify; MARGIN: 0pt 0px; FONT: 10pt Times New Roman, Times, Serif"&gt;  The Company capitalizes costs associated with the issuance of debt  instruments and amortizes these costs to interest expense over the  term of the related debt agreement using the effective yield  amortization method. Unamortized debt issuance costs will be  charged to operations when indebtedness under the related credit  facility is repaid prior to maturity.&lt;/div&gt;    &lt;div style="clear:both; FONT-FAMILY:Times New Roman;FONT-SIZE: 10pt"&gt;  &amp;#160;&lt;/div&gt;    &lt;div style="clear:both; FONT-FAMILY:Times New Roman;FONT-SIZE: 10pt;TEXT-ALIGN: justify; MARGIN: 0pt 0px; FONT: 10pt Times New Roman, Times, Serif"&gt;  &lt;b&gt;&lt;i&gt;Business Acquisitions&lt;/i&gt;&lt;/b&gt;&lt;/div&gt;    &lt;div style="clear:both; FONT-FAMILY:Times New Roman;FONT-SIZE: 10pt;TEXT-ALIGN: justify; MARGIN: 0pt 0px; FONT: 10pt Times New Roman, Times, Serif"&gt;  &amp;#160;&lt;/div&gt;    &lt;div style="clear:both; FONT-FAMILY:Times New Roman;FONT-SIZE: 10pt;TEXT-ALIGN: justify; MARGIN: 0pt 0px; FONT: 10pt Times New Roman, Times, Serif"&gt;  Business acquisitions are accounted for using the acquisition  method of accounting in accordance with Accounting Standards  Codification (&amp;#8220;ASC&amp;#8221;) 805, &amp;#8220;Business  Combinations.&amp;#8221; ASC 805 requires, among other things, that  assets acquired and liabilities assumed be recognized at their fair  values, as determined in accordance with ASC 820, &amp;#8220;Fair Value  Measurements,&amp;#8221; as of the acquisition date. For certain assets  and liabilities, book value approximates fair value. In addition,  ASC 805 establishes that consideration transferred be measured at  the closing date of the acquisition at the then-current market  price, which may be different than the amount of consideration  assumed in the pro forma financial statements. Under ASC 805,  acquisition related costs (i.e., advisory, legal, valuation and  other professional fees) and certain acquisition-related  restructuring charges impacting the target company are expensed in  the period in which the costs are incurred. The application of the  acquisition method of accounting requires&amp;#160;the Company&amp;#160;to  make estimates and assumptions related to the estimated fair values  of net assets acquired. Significant judgments are used during this  process, particularly with respect to intangible assets. Generally,  intangible assets are amortized over their estimated useful lives.  Goodwill and other indefinite-lived intangibles are not amortized,  but are annually assessed for impairment. Therefore, the purchase  price allocation to intangible assets and goodwill has a  significant impact on future operating results.&lt;/div&gt;    &lt;div style="clear:both; FONT-FAMILY:Times New Roman;FONT-SIZE: 10pt;TEXT-ALIGN: justify; MARGIN: 0pt 0px; FONT: 10pt Times New Roman, Times, Serif"&gt;  &lt;b&gt;&lt;i&gt;&amp;#160;&lt;/i&gt;&lt;/b&gt;&lt;/div&gt;    &lt;div style="clear:both; FONT-FAMILY:Times New Roman;FONT-SIZE: 10pt;TEXT-ALIGN: justify; MARGIN: 0pt 0px; FONT: 10pt Times New Roman, Times, Serif"&gt;  &lt;b&gt;&lt;i&gt;Reclassification&lt;/i&gt;&lt;/b&gt;&lt;/div&gt;    &lt;div style="clear:both; FONT-FAMILY:Times New Roman;FONT-SIZE: 10pt;TEXT-ALIGN: justify; MARGIN: 0pt 0px; FONT: 10pt Times New Roman, Times, Serif"&gt;  &amp;#160;&lt;/div&gt;    &lt;div style="clear:both; FONT-FAMILY:Times New Roman;FONT-SIZE: 10pt;TEXT-ALIGN: justify; MARGIN: 0pt 0px; FONT: 10pt Times New Roman, Times, Serif"&gt;  Certain balances in the prior year&lt;font style="FONT-FAMILY: 'Times New Roman'; FONT-SIZE: 12pt"&gt;&lt;font size="2"&gt;&amp;#8217;&lt;/font&gt;&lt;/font&gt;s consolidated financial statements have  been reclassified to conform to the June 30, 2013 presentation.  These changes consisted of a reclassification on the consolidated  statements of operations from general and administrative expense to  a separate line item entitled charges related to U.S. Government  settlement and a reclassification on the consolidated statements of  cash flows from changes in accounts payable and accrued expenses to  a separate line item entitled increase (decrease) in accrued  expenses related to U.S. Government settlement.&lt;/div&gt;    &lt;div style="clear:both; FONT-FAMILY:Times New Roman;FONT-SIZE: 10pt;TEXT-ALIGN: justify; MARGIN: 0pt 0px; FONT: 10pt Times New Roman, Times, Serif"&gt;  &amp;#160;&lt;/div&gt;    &lt;div style="clear:both; FONT-FAMILY:Times New Roman;FONT-SIZE: 10pt;TEXT-ALIGN: justify; MARGIN: 0pt 0px; FONT: 10pt Times New Roman, Times, Serif"&gt;  &lt;b&gt;Impact of Recently Issued Accounting Standards&lt;/b&gt;&lt;/div&gt;    &lt;div style="clear:both; FONT-FAMILY:Times New Roman;FONT-SIZE: 10pt;TEXT-ALIGN: justify; MARGIN: 0pt 0px; FONT: 10pt Times New Roman, Times, Serif"&gt;  &lt;b&gt;&amp;#160;&lt;/b&gt;&lt;/div&gt;    &lt;div style="clear:both; FONT-FAMILY:Times New Roman;FONT-SIZE: 10pt;TEXT-ALIGN: justify; MARGIN: 0pt 0px; FONT: 10pt Times New Roman, Times, Serif"&gt;  In February 2013, the Financial Accounting Standards Board issued  guidance (ASU No. 2013-02) finalizing the reporting of amounts  reclassified out of accumulated other comprehensive income. The new  standard requires either in a single note or parenthetically on the  face of the financial statements, the effect of significant amounts  reclassified from each component of accumulated other comprehensive  income based on its source and the income statement line items  affected by the reclassification. The guidance is effective for  annual reporting periods and interim periods within those years,  beginning after December 15, 2012. In the first quarter of 2013,  the Company adopted the guidance and determined that there were no  amounts reclassified in the period that would require enhanced  disclosure.&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 all significant accounting policies of the reporting entity.</ElementDefenition><ElementReferences>Reference 1: http://www.xbrl.org/2003/role/presentationRef

 -Publisher FASB

 -Name Accounting Standards Codification

 -Topic 235

 -SubTopic 10

 -Section 50

 -Paragraph 3

 -URI http://asc.fasb.org/extlink&amp;oid=6367646&amp;loc=d3e18780-107790



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

 -Publisher FASB

 -Name Accounting Standards Codification

 -Topic 235

 -SubTopic 10

 -Section 50

 -Paragraph 1

 -URI http://asc.fasb.org/extlink&amp;oid=6367646&amp;loc=d3e18726-107790



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

 -Publisher FASB

 -Name Accounting Standards Codification

 -Topic 235

 -SubTopic 10

 -Section 50

 -Paragraph 6

 -URI http://asc.fasb.org/extlink&amp;oid=6367646&amp;loc=d3e18861-107790



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

 -Publisher FASB

 -Name Accounting Standards Codification

 -Topic 235

 -SubTopic 10

 -Section 50

 -Paragraph 2

 -URI http://asc.fasb.org/extlink&amp;oid=6367646&amp;loc=d3e18743-107790



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

 -Publisher FASB

 -Name Accounting Standards Codification

 -Topic 235

 -SubTopic 10

 -Section 50

 -Paragraph 5

 -URI http://asc.fasb.org/extlink&amp;oid=6367646&amp;loc=d3e18854-107790



</ElementReferences><IsTotalLabel>false</IsTotalLabel><UnitID>0</UnitID><Label>Significant Accounting Policies [Text Block]</Label></Row></Rows><Footnotes /><IsEquityReport>false</IsEquityReport><ReportName>Summary of Significant Accounting Policies</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://www.baxanosurgical.com/role/SummaryOfSignificantAccountingPolicies</RoleURI><NumberOfCols>1</NumberOfCols><NumberOfRows>2</NumberOfRows></InstanceReport>
