<?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>109 - 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>eol_PE631979--13S-3-0008_STD_90_20130331_0</ContextID><EntitySchema>http://www.sec.gov/CIK</EntitySchema><EntityValue>0001287865</EntityValue><PeriodDisplayName /><PeriodType>duration</PeriodType><PeriodStartDate>2013-01-01T00:00:00</PeriodStartDate><PeriodEndDate>2013-03-31T00:00:00</PeriodEndDate><Segments /><Scenarios /></contextRef><UPS /><CurrencyCode /><OriginalCurrencyCode /></MCU><CurrencySymbol /><Labels><Label Key="CalendarSupplement" Id="0" Label="3 Months Ended" /><Label Key="Calendar" Id="1" Label="Mar. 31, 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>terseLabel</PreferredLabelRole><FootnoteIndexer /><Cells><Cell FlagID="0" ContextID="eol_PE631979--13S-3-0008_STD_90_20130331_0" UnitID=""><Id>1</Id><IsNumeric>false</IsNumeric><IsRatio>false</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>0</NumericAmount><RoundedNumericAmount>0</RoundedNumericAmount><NonNumbericText>&lt;div&gt;
&lt;p style="MARGIN-TOP: 18px; MARGIN-BOTTOM: 0px"&gt;&lt;font style="FONT-FAMILY: Times New Roman" size="2"&gt;&lt;b&gt;2. Summary of
Significant Accounting Policies&lt;/b&gt;&lt;/font&gt;&lt;/p&gt;
&lt;p style="MARGIN-TOP: 6px; MARGIN-BOTTOM: 0px"&gt;&lt;font style="FONT-FAMILY: Times New Roman" size="2"&gt;&lt;i&gt;Unaudited Interim
Condensed Consolidated Financial Statements&lt;/i&gt;: The accompanying
unaudited interim condensed consolidated financial statements have
been prepared in accordance with accounting principles generally
accepted in the United States for interim financial information,
including rules and regulations of the Securities and Exchange
Commission. Accordingly, they do not include all of the information
and footnotes required by generally accepted accounting principles
for complete financial statements. In the opinion of management,
all adjustments (consisting of normal recurring accruals)
considered necessary for a fair presentation have been included.
Operating results for the three month period ended March&amp;#xA0;31,
2013, are not necessarily indicative of the results that may be
expected for the year ending December&amp;#xA0;31, 2013. The condensed
consolidated balance sheet at December&amp;#xA0;31, 2012 has been
derived from the audited financial statements at that date but does
not include all of the information and footnotes required by
accounting principles generally accepted in the United States for
complete financial statements.&lt;/font&gt;&lt;/p&gt;
&lt;p style="MARGIN-TOP: 12px; MARGIN-BOTTOM: 0px"&gt;&lt;font style="FONT-FAMILY: Times New Roman" size="2"&gt;For information about
significant accounting policies, refer to the consolidated
financial statements and footnotes thereto included in our Annual
Report on Form 10-K for the year ended December&amp;#xA0;31, 2012.
During the three months ended March&amp;#xA0;31, 2013, there were no
material changes to these policies.&lt;/font&gt;&lt;/p&gt;
&lt;p style="MARGIN-TOP: 12px; MARGIN-BOTTOM: 0px"&gt;&lt;font style="FONT-FAMILY: Times New Roman" size="2"&gt;&lt;i&gt;Reclassifications:&lt;/i&gt;
Certain reclassifications have been made to the condensed
consolidated financial statements to conform to the 2013
consolidated financial statement presentation. These
reclassifications had no impact on stockholders&amp;#x2019; equity or
net income.&lt;/font&gt;&lt;/p&gt;
&lt;p style="MARGIN-TOP: 18px; MARGIN-BOTTOM: 0px"&gt;&lt;font style="FONT-FAMILY: Times New Roman" size="2"&gt;&lt;i&gt;Variable Interest
Entities&lt;/i&gt;&lt;/font&gt;&lt;/p&gt;
&lt;p style="MARGIN-TOP: 6px; MARGIN-BOTTOM: 0px"&gt;&lt;font style="FONT-FAMILY: Times New Roman" size="2"&gt;At March 31, 2013, we had
loans to and/or equity investments in several variable interest
entities (&amp;#x201C;VIEs&amp;#x201D;) for which we are not the primary
beneficiary. The carrying value and classification of the related
assets and maximum exposure to loss as a result of our involvement
with these VIEs are presented below at March 31, 2013 (in
thousands):&lt;/font&gt;&lt;/p&gt;
&lt;p style="MARGIN-TOP: 0px; MARGIN-BOTTOM: 0px; FONT-SIZE: 12px"&gt;
&amp;#xA0;&lt;/p&gt;
&lt;table style="BORDER-COLLAPSE: collapse" border="0" cellspacing="0" cellpadding="0" width="84%" align="center"&gt;
&lt;tr&gt;
&lt;td width="51%"&gt;&lt;/td&gt;
&lt;td valign="bottom" width="5%"&gt;&lt;/td&gt;
&lt;td&gt;&lt;/td&gt;
&lt;td&gt;&lt;/td&gt;
&lt;td&gt;&lt;/td&gt;
&lt;td valign="bottom" width="5%"&gt;&lt;/td&gt;
&lt;td&gt;&lt;/td&gt;
&lt;td valign="bottom" width="5%"&gt;&lt;/td&gt;
&lt;td&gt;&lt;/td&gt;
&lt;td&gt;&lt;/td&gt;
&lt;td&gt;&lt;/td&gt;
&lt;/tr&gt;
&lt;tr&gt;
&lt;td valign="bottom" nowrap="nowrap"&gt;
&lt;p style="BORDER-BOTTOM: #000000 1px solid; WIDTH: 33pt"&gt;
&lt;font style="FONT-FAMILY: Times New Roman" size="1"&gt;&lt;b&gt;VIE
Type&lt;/b&gt;&lt;/font&gt;&lt;/p&gt;
&lt;/td&gt;
&lt;td valign="bottom"&gt;&lt;font size="1"&gt;&amp;#xA0;&amp;#xA0;&lt;/font&gt;&lt;/td&gt;
&lt;td style="BORDER-BOTTOM: #000000 1px solid" valign="bottom" colspan="2" align="center"&gt;&lt;font style="FONT-FAMILY: Times New Roman" size="1"&gt;&lt;b&gt;Maximum&amp;#xA0;Loss&lt;br /&gt;
Exposure(1)&lt;/b&gt;&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom"&gt;&lt;font size="1"&gt;&amp;#xA0;&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom"&gt;&lt;font size="1"&gt;&amp;#xA0;&amp;#xA0;&lt;/font&gt;&lt;/td&gt;
&lt;td style="BORDER-BOTTOM: #000000 1px solid" valign="bottom" align="center"&gt;&lt;font style="FONT-FAMILY: Times New Roman" size="1"&gt;&lt;b&gt;Asset&amp;#xA0;Type&lt;/b&gt;&lt;/font&gt;&lt;br /&gt;
&lt;font style="FONT-FAMILY: Times New Roman" size="1"&gt;&lt;b&gt;Classification&lt;/b&gt;&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom"&gt;&lt;font size="1"&gt;&amp;#xA0;&amp;#xA0;&lt;/font&gt;&lt;/td&gt;
&lt;td style="BORDER-BOTTOM: #000000 1px solid" valign="bottom" colspan="2" align="center"&gt;&lt;font style="FONT-FAMILY: Times New Roman" size="1"&gt;&lt;b&gt;Carrying&lt;br /&gt;
Amount(2)&lt;/b&gt;&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom"&gt;&lt;font size="1"&gt;&amp;#xA0;&lt;/font&gt;&lt;/td&gt;
&lt;/tr&gt;
&lt;tr bgcolor="#CCEEFF"&gt;
&lt;td valign="top"&gt;
&lt;p style="TEXT-INDENT: -1em; MARGIN-LEFT: 1em"&gt;&lt;font style="FONT-FAMILY: Times New Roman" size="2"&gt;Loans, net&lt;/font&gt;&lt;/p&gt;
&lt;/td&gt;
&lt;td valign="bottom"&gt;&lt;font size="1"&gt;&amp;#xA0;&amp;#xA0;&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom"&gt;&lt;font style="FONT-FAMILY: Times New Roman" size="2"&gt;$&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom" align="right"&gt;&lt;font style="FONT-FAMILY: Times New Roman" size="2"&gt;275,390&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom" nowrap="nowrap"&gt;&lt;font style="FONT-FAMILY: Times New Roman" size="2"&gt;&amp;#xA0;&amp;#xA0;&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom"&gt;&lt;font size="1"&gt;&amp;#xA0;&amp;#xA0;&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom" align="center"&gt;&lt;font style="FONT-FAMILY: Times New Roman" size="2"&gt;Mortgage&amp;#xA0;and&amp;#xA0;other&amp;#xA0;loans&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom"&gt;&lt;font size="1"&gt;&amp;#xA0;&amp;#xA0;&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom"&gt;&lt;font style="FONT-FAMILY: Times New Roman" size="2"&gt;$&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom" align="right"&gt;&lt;font style="FONT-FAMILY: Times New Roman" size="2"&gt;226,891&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom" nowrap="nowrap"&gt;&lt;font style="FONT-FAMILY: Times New Roman" size="2"&gt;&amp;#xA0;&amp;#xA0;&lt;/font&gt;&lt;/td&gt;
&lt;/tr&gt;
&lt;tr&gt;
&lt;td valign="top"&gt;
&lt;p style="TEXT-INDENT: -1em; MARGIN-LEFT: 1em"&gt;&lt;font style="FONT-FAMILY: Times New Roman" size="2"&gt;Equity
investments&lt;/font&gt;&lt;/p&gt;
&lt;/td&gt;
&lt;td valign="bottom"&gt;&lt;font size="1"&gt;&amp;#xA0;&amp;#xA0;&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom"&gt;&lt;font style="FONT-FAMILY: Times New Roman" size="2"&gt;$&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom" align="right"&gt;&lt;font style="FONT-FAMILY: Times New Roman" size="2"&gt;18,928&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom" nowrap="nowrap"&gt;&lt;font style="FONT-FAMILY: Times New Roman" size="2"&gt;&amp;#xA0;&amp;#xA0;&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom"&gt;&lt;font size="1"&gt;&amp;#xA0;&amp;#xA0;&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom" align="center"&gt;&lt;font style="FONT-FAMILY: Times New Roman" size="2"&gt;Other&amp;#xA0;assets&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom"&gt;&lt;font size="1"&gt;&amp;#xA0;&amp;#xA0;&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom"&gt;&lt;font style="FONT-FAMILY: Times New Roman" size="2"&gt;$&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom" align="right"&gt;&lt;font style="FONT-FAMILY: Times New Roman" size="2"&gt;5,374&lt;/font&gt;&lt;/td&gt;
&lt;td valign="bottom" nowrap="nowrap"&gt;&lt;font style="FONT-FAMILY: Times New Roman" size="2"&gt;&amp;#xA0;&amp;#xA0;&lt;/font&gt;&lt;/td&gt;
&lt;/tr&gt;
&lt;/table&gt;
&lt;p style="MARGIN-TOP: 0px; MARGIN-BOTTOM: 0px; FONT-SIZE: 12px"&gt;
&amp;#xA0;&lt;/p&gt;
&lt;table style="BORDER-COLLAPSE: collapse" border="0" cellspacing="0" cellpadding="0" width="100%"&gt;
&lt;tr&gt;
&lt;td valign="top" width="4%" align="left"&gt;&lt;font style="FONT-FAMILY: Times New Roman" size="2"&gt;(1)&lt;/font&gt;&lt;/td&gt;
&lt;td valign="top" align="left"&gt;&lt;font style="FONT-FAMILY: Times New Roman" size="2"&gt;Our maximum loss exposure
related to loans with VIEs represents our current aggregate gross
carrying value of the loan plus accrued interest and any other
related assets (such as rents receivable), less any liabilities.
Our maximum loss exposure related to our equity investment in VIEs
represents the current carrying values of such investment plus any
other related assets (such as rent receivables) less any
liabilities.&lt;/font&gt;&lt;/td&gt;
&lt;/tr&gt;
&lt;/table&gt;
&lt;table style="BORDER-COLLAPSE: collapse" border="0" cellspacing="0" cellpadding="0" width="100%"&gt;
&lt;tr&gt;
&lt;td valign="top" width="4%" align="left"&gt;&lt;font style="FONT-FAMILY: Times New Roman" size="2"&gt;(2)&lt;/font&gt;&lt;/td&gt;
&lt;td valign="top" align="left"&gt;&lt;font style="FONT-FAMILY: Times New Roman" size="2"&gt;Carrying amount reflects
the net book value of our loan or equity interest only in the
VIE.&lt;/font&gt;&lt;/td&gt;
&lt;/tr&gt;
&lt;/table&gt;
&lt;p style="MARGIN-TOP: 12px; MARGIN-BOTTOM: 0px"&gt;&lt;font style="FONT-FAMILY: Times New Roman" size="2"&gt;For the VIE types above, we
do not consolidate the VIE because we do not have the ability to
control the activities (such as the day-to-day healthcare
operations of our borrower or investee) that most significantly
impact the VIE&amp;#x2019;s economic performance. As of March 31, 2013,
we were not required to provide financial support through a
liquidity arrangement or otherwise to our unconsolidated VIEs,
including circumstances in which it could be exposed to further
losses (e.g., cash short falls).&lt;/font&gt;&lt;/p&gt;
&lt;p style="MARGIN-TOP: 12px; MARGIN-BOTTOM: 0px"&gt;&lt;font style="FONT-FAMILY: Times New Roman" size="2"&gt;Typically, our loans are
collateralized by assets of the borrower (some assets of which are
on the premises of facilities owned by us) and further supported by
limited guarantees made by certain principals of the
borrower.&lt;/font&gt;&lt;/p&gt;
&lt;p style="MARGIN-TOP: 12px; MARGIN-BOTTOM: 0px"&gt;&lt;font style="FONT-FAMILY: Times New Roman" size="2"&gt;See Note 3 for additional
description of the nature, purpose and activities of our more
significant VIEs and interests therein.&lt;/font&gt;&lt;/p&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 AICPA

 -Name Accounting Principles Board Opinion (APB)

 -Number 22

 -Paragraph 8

 -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 4: 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 5: 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 6: 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>Summary of Significant Accounting Policies</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.medicalpropertiestrust.com/taxonomy/role/NotesToFinancialStatementsSignificantAccountingPoliciesTextBlock</RoleURI><NumberOfCols>1</NumberOfCols><NumberOfRows>2</NumberOfRows></InstanceReport>
