﻿<?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>Significant Accounting Policies</ReportName>
  <ReportLongName>01010 - Disclosure - 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="9 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>EPS</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>Disclosure - Significant Accounting Policies [Abstract]</Label>
      <Level>0</Level>
      <ElementName>pre_DisclosureSignificantAccountingPoliciesAbstract</ElementName>
      <ElementPrefix>pre</ElementPrefix>
      <IsBaseElement>false</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>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 />
      <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;p style='margin-top:13.5pt; margin-bottom:0pt'&gt;&lt;font style="font-family:Times New Roman;font-size:10pt;font-weight:bold;margin-left:0px;"&gt;2. Significant Accounting Policies &lt;/font&gt;&lt;/p&gt;&lt;p style='margin-top:4.5pt; margin-bottom:0pt'&gt;&lt;font style="font-family:Times New Roman;font-size:10pt;margin-left:0px;"&gt;The Company's Unaudited Condensed Consolidated Financial Statements have been prepared in accordance with accounting principles generally accepted in the United States (U.S. GAAP) for interim financial information and with the instructions for Form 10-Q and Article 10 of Regulation S-X. The Unaudited Condensed Consolidated Financial Statements include the accounts of the Company and its subsidiaries. Intercompany accounts and transactions have been eliminated. &lt;/font&gt;&lt;/p&gt;&lt;p style='margin-top:9pt; margin-bottom:0pt'&gt;&lt;font style="font-family:Times New Roman;font-size:10pt;margin-left:0px;"&gt;The preparation of financial statements in conformity with U.S. GAAP requires Management to make estimates and assumptions that affect the reported amounts of assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. While Management believes that the amounts included in the Unaudited Condensed Consolidated Financial Statements reflect its best estimates and assumptions, actual results could differ from those estimates. The Company's principal estimates include: &lt;/font&gt;&lt;/p&gt;&lt;p style='margin-top:4.5pt; margin-bottom:0pt'&gt;&lt;font style="font-family:Times New Roman;font-size:10pt;margin-left:24.45px;"&gt;&amp;#8226; &amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;Unpaid losses and loss expenses; &lt;/font&gt;&lt;/p&gt;&lt;p style='margin-top:4.5pt; margin-bottom:0pt'&gt;&lt;font style="font-family:Times New Roman;font-size:10pt;margin-left:24.45px;"&gt;&amp;#8226; &amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;Policy benefits for life and annuity contracts; &lt;/font&gt;&lt;/p&gt;&lt;p style='margin-top:4.5pt; margin-bottom:0pt'&gt;&lt;font style="font-family:Times New Roman;font-size:10pt;margin-left:24.45px;"&gt;&amp;#8226; &amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;Gross and net premiums written and net premiums earned; &lt;/font&gt;&lt;/p&gt;&lt;p style='margin-top:4.5pt; margin-bottom:0pt'&gt;&lt;font style="font-family:Times New Roman;font-size:10pt;margin-left:24.45px;"&gt;&amp;#8226; &amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;Recoverability of deferred acquisition costs; &lt;/font&gt;&lt;/p&gt;&lt;p style='margin-top:4.5pt; margin-bottom:0pt'&gt;&lt;font style="font-family:Times New Roman;font-size:10pt;margin-left:24.45px;"&gt;&amp;#8226; &amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;Recoverability of deferred tax assets; &lt;/font&gt;&lt;/p&gt;&lt;p style='margin-top:4.5pt; margin-bottom:0pt'&gt;&lt;font style="font-family:Times New Roman;font-size:10pt;margin-left:24.45px;"&gt;&amp;#8226; &amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;Valuation of goodwill and intangible assets; and &lt;/font&gt;&lt;/p&gt;&lt;p style='margin-top:4.5pt; margin-bottom:0pt'&gt;&lt;font style="font-family:Times New Roman;font-size:10pt;margin-left:24.45px;"&gt;&amp;#8226; &amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;&amp;#160;Valuation of certain assets and derivative financial instruments that are measured using significant unobservable inputs. &lt;/font&gt;&lt;/p&gt;&lt;p style='margin-top:9pt; margin-bottom:0pt'&gt;&lt;font style="font-family:Times New Roman;font-size:10pt;margin-left:0px;"&gt;In the opinion of Management, all adjustments (which include normal recurring adjustments) necessary for a fair presentation of results for the interim periods have been made. As the Company's reinsurance operations are exposed to low-frequency, high-severity risk events, some of which are seasonal, results for certain interim periods may include unusually low loss experience, while results for other interim periods may include significant catastrophic losses. Consequently, the Company's results for interim periods are not necessarily indicative of results for the full year. These Unaudited Condensed Consolidated Financial Statements should be read in conjunction with the Consolidated Financial Statements and notes thereto included in the Company's Annual Report on Form 10-K for the year ended December&amp;#160;31, 2009. &lt;/font&gt;&lt;/p&gt;&lt;p style='margin-top:9pt; margin-bottom:0pt'&gt;&lt;font style="font-family:Times New Roman;font-size:10pt;margin-left:0px;"&gt;The following significant accounting policies were adopted by the Company during the &lt;/font&gt;&lt;font style="font-family:Times New Roman;font-size:10pt;"&gt;nine &lt;/font&gt;&lt;font style="font-family:Times New Roman;font-size:10pt;"&gt;months ended &lt;/font&gt;&lt;font style="font-family:Times New Roman;font-size:10pt;"&gt;September 30, 2010&lt;/font&gt;&lt;font style="font-family:Times New Roman;font-size:10pt;"&gt;. The adoption of these policies did not have an impact on the Company's consolidated shareholders' equity or net income. &lt;/font&gt;&lt;/p&gt;&lt;p style='margin-top:9pt; margin-bottom:0pt'&gt;&lt;/p&gt;&lt;ul&gt;&lt;li style="margin-left:36px;list-style:disc;"&gt;&lt;font style="font-family:Times New Roman;font-size:10pt;"&gt;In June 2009, the Financial Accounting Standards Board (FASB) issued new accounting guidance which replaced the quantitative approach previously required for determining the primary beneficiary of a variable interest entity (VIE) with a qualitative approach to determine whether the Company has a controlling interest (primary beneficiary) in a VIE at the date when it becomes initially involved in the VIE. The guidance also requires the Company to perform ongoing reassessments, and provide certain disclosures, related to its involvement in VIEs. The Company adopted this guidance as of January 1, 2010.&lt;/font&gt;&lt;/li&gt;&lt;/ul&gt;&lt;p style='margin-top:6pt; margin-bottom:0pt'&gt;&lt;font style="font-family:Times New Roman;font-size:10pt;margin-left:36px;"&gt;The Company is involved in the normal course of business with VIEs as a passive investor in certain asset-backed securities, other fixed maturity investments and limited partnerships, that are issued by third party VIEs. The Company's maximum exposure to loss with respect to these investments is limited to the investment carrying amounts reported in the Company's Unaudited Condensed Consolidated Balance Sheets and any unfunded commitments. The Company also has three indirect wholly-owned subsidiaries that are considered to be VIEs, which were utilized to issue the Company's Senior Notes and Capital Efficient Notes (see Note 16 to Consolidated Financial Statements in the Company's Annual Report on Form 10-K for the year ended December 31, 2009). The Company determined that it was not the primary beneficiary of any of these VIEs as of &lt;/font&gt;&lt;font style="font-family:Times New Roman;font-size:10pt;"&gt;September 30, 2010&lt;/font&gt;&lt;font style="font-family:Times New Roman;font-size:10pt;"&gt;.&lt;/font&gt;&lt;/p&gt;&lt;p style='margin-top:6pt; margin-bottom:0pt'&gt;&lt;/p&gt;&lt;ul&gt;&lt;li style="margin-left:36px;list-style:disc;"&gt;&lt;font style="font-family:Times New Roman;font-size:10pt;"&gt;In January 2010, the FASB issued new accounting guidance which requires the Company to disclose additional information about its fair value measurements at a greater level of disaggregation. The additional disclosures include information about significant transfers into and/or out of the Level 1 and 2 categories, other disclosures about inputs and valuation techniques, and expanded disclosures related to the Level 3 activity. The Company adopted the guidance related to disclosures at a greater level of disaggregation, disclosures about transfers into and/or out of the Level 1 and 2 categories and other disclosures about inputs and valuation techniques as of January&amp;#160;1, 2010. Expanded disclosures related to the Level 3 activity will be effective for in&lt;/font&gt;&lt;font style="font-family:Times New Roman;font-size:10pt;"&gt;terim and annual periods beginni&lt;/font&gt;&lt;font style="font-family:Times New Roman;font-size:10pt;"&gt;ng after December&amp;#160;15, 2010.&lt;/font&gt;&lt;/li&gt;&lt;li style="margin-left:36px;list-style:disc;"&gt;&lt;font style="font-family:Times New Roman;font-size:10pt;"&gt;In March 2010, the FASB issued new accounting guidance for embedded credit derivatives, which clarifies that only a credit derivative related to the subordination of one financial instrument to another is exempt from embedded derivative bifurcation requirements. As a result, entities that have contracts containing an embedded credit derivative feature in a form other than such subordination may need to separately account for the embedded credit derivative feature. The Company adopted this guidance as of July 1, 2010.&lt;/font&gt;&lt;font style="font-family:Times New Roman;font-size:10pt;"&gt; &lt;/font&gt;&lt;/li&gt;&lt;/ul&gt;</NonNumbericText>
          <NonNumericTextHeader>2. Significant Accounting Policies The Company's Unaudited Condensed Consolidated Financial Statements have been prepared in accordance with accounting</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>
