<?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>108 - Disclosure - Basis of Presentation and 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_PE746391--1310-Q0003_STD_181_20130630_0</ContextID><EntitySchema>http://www.sec.gov/CIK</EntitySchema><EntityValue>0001401708</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_BasisOfPresentationAndSignificantAccountingPoliciesTextBlock</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_PE746391--1310-Q0003_STD_181_20130630_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. Basis of Presentation
and 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;b&gt;&lt;i&gt;Basis of
Presentation&lt;/i&gt;&lt;/b&gt;&lt;/font&gt;&lt;/p&gt;
&lt;p style="MARGIN-TOP: 6px; TEXT-INDENT: 4%; MARGIN-BOTTOM: 0px"&gt;
&lt;font style="FONT-FAMILY: Times New Roman" size="2"&gt;The
accompanying unaudited condensed consolidated financial statements
reflect the accounts of the Company and its wholly-owned
subsidiaries, NanoString Technologies International, Inc.,
NanoString Technologies Asia Pacific Limited, NanoString
Technologies Europe Limited and NanoString Technologies SAS. The
condensed consolidated balance sheet data as of December&amp;#xA0;31,
2012 were derived from audited financial statements not included in
this report. The accompanying unaudited condensed consolidated
financial statements have been prepared in accordance with the
rules and regulations of the Securities and Exchange Commission
(&amp;#x201C;SEC&amp;#x201D;) and generally accepted accounting principles in
the United States of America (&amp;#x201C;GAAP&amp;#x201D;) for unaudited
condensed consolidated financial information. Accordingly, they do
not include all of the information and footnotes required by GAAP
for complete financial statements. The accompanying unaudited
condensed consolidated financial statements reflect all adjustments
consisting of normal recurring adjustments which, in the opinion of
management, are necessary for a fair statement of the
Company&amp;#x2019;s financial position and results of its operations,
as of and for the periods presented.&lt;/font&gt;&lt;/p&gt;
&lt;p style="MARGIN-TOP: 12px; TEXT-INDENT: 4%; MARGIN-BOTTOM: 0px"&gt;
&lt;font style="FONT-FAMILY: Times New Roman" size="2"&gt;Unless
indicated otherwise, all amounts presented in financial tables are
presented in thousands, except for per share and par value
amounts.&lt;/font&gt;&lt;/p&gt;
&lt;p style="MARGIN-TOP: 12px; TEXT-INDENT: 4%; MARGIN-BOTTOM: 0px"&gt;
&lt;font style="FONT-FAMILY: Times New Roman" size="2"&gt;These unaudited
condensed consolidated financial statements should be read in
conjunction with the audited consolidated financial statements and
accompanying notes included in the prospectus that forms a part of
the Company&amp;#x2019;s Registration Statement on Form&amp;#xA0;S-1 (File
No.&amp;#xA0;333-188704), which prospectus was filed with the SEC
pursuant to Rule&amp;#xA0;424 promulgated under the Securities Act of
1933 on June&amp;#xA0;26, 2013.&lt;/font&gt;&lt;/p&gt;
&lt;p style="MARGIN-TOP: 12px; TEXT-INDENT: 4%; MARGIN-BOTTOM: 0px"&gt;
&lt;font style="FONT-FAMILY: Times New Roman" size="2"&gt;The preparation
of financial statements in conformity with GAAP requires management
to make estimates and assumptions that affect the amounts reported
in the condensed consolidated financial statements and accompanying
notes. Actual results could differ from those estimates. The
results of the Company&amp;#x2019;s operations for the three and six
month periods ended June&amp;#xA0;30, 2013 are not necessarily
indicative of the results to be expected for the full year or for
any other period.&lt;/font&gt;&lt;/p&gt;
&lt;p style="MARGIN-TOP: 18px; MARGIN-BOTTOM: 0px; FONT-SIZE: 1px"&gt;
&amp;#xA0;&lt;/p&gt;
&lt;p style="MARGIN-TOP: 0px; MARGIN-BOTTOM: 0px"&gt;&lt;font style="FONT-FAMILY: Times New Roman" size="2"&gt;&lt;b&gt;&lt;i&gt;Revenue
Recognition&lt;/i&gt;&lt;/b&gt;&lt;/font&gt;&lt;/p&gt;
&lt;p style="MARGIN-TOP: 6px; TEXT-INDENT: 4%; MARGIN-BOTTOM: 0px"&gt;
&lt;font style="FONT-FAMILY: Times New Roman" size="2"&gt;The Company
recognizes revenue when (1)&amp;#xA0;persuasive evidence of an
arrangement exists, (2)&amp;#xA0;delivery has occurred or services have
been rendered, (3)&amp;#xA0;the price to the customer is fixed or
determinable and (4)&amp;#xA0;collectability is reasonably assured. The
Company generates revenue from the sale of products and services.
The Company&amp;#x2019;s products consist of its proprietary nCounter
Analysis System and related consumables. Services consist of
extended warranties and service fees for assay processing. A
delivered product or service is considered to be a separate unit of
accounting when it has value to the customer on a stand-alone
basis. Products or services have value on a stand-alone basis if
they are sold separately by any vendor or the customer could resell
the delivered product.&lt;/font&gt;&lt;/p&gt;
&lt;p style="MARGIN-TOP: 12px; TEXT-INDENT: 4%; MARGIN-BOTTOM: 0px"&gt;
&lt;font style="FONT-FAMILY: Times New Roman" size="2"&gt;Systems product
revenue is recognized upon installation and calibration in
geographic regions where such services are only available from
specialized Company technicians. In these regions, systems and
related installation and calibration are considered to be one unit
of accounting, as systems are required to be professionally
installed and calibrated before use. In certain geographic regions,
installation and calibration services are available from other
vendors, and in such regions they are considered separate revenue
elements. Consumables are considered to be separate units of
accounting as they are sold separately. Consumables product revenue
is recognized upon shipment.&lt;/font&gt;&lt;/p&gt;
&lt;p style="MARGIN-TOP: 12px; TEXT-INDENT: 4%; MARGIN-BOTTOM: 0px"&gt;
&lt;font style="FONT-FAMILY: Times New Roman" size="2"&gt;Service revenue
is recognized when earned, which is generally upon the rendering of
the related services. Extended warranties and service fees for
assay processing are each considered separate units of accounting
as they are sold separately. The Company offers extended warranties
on its nCounter Analysis System for periods ranging from 12 to 36
months after the end of the standard 12-month warranty period.
Extended warranties are generally separately priced. Revenue from
extended warranties are deferred and recognized in income on a
straight-line basis over the warranty period.&lt;/font&gt;&lt;/p&gt;
&lt;p style="MARGIN-TOP: 12px; TEXT-INDENT: 4%; MARGIN-BOTTOM: 0px"&gt;
&lt;font style="FONT-FAMILY: Times New Roman" size="2"&gt;The Company
allocates the contract consideration at the inception of the
contract to the deliverables based upon their relative selling
prices. To date, selling prices have been established by reference
to vendor specific objective evidence based on stand-alone sales
transactions for each deliverable. Vendor specific objective
evidence is considered to have been established when a substantial
majority of individual sales transactions within the previous 12
month period fall within a reasonably narrow range, which the
Company has defined to be plus or minus 15% of the median sales
price of actual stand-alone sales transactions. The Company uses
its best estimate of selling price for individual deliverables when
vendor specific objective evidence or third-party evidence is
unavailable. Allocated revenue is only recognized for each
deliverable when the revenue recognition criteria have been
met.&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;b&gt;&lt;i&gt;Recent Accounting
Pronouncements&lt;/i&gt;&lt;/b&gt;&lt;/font&gt;&lt;/p&gt;
&lt;p style="MARGIN-TOP: 6px; TEXT-INDENT: 4%; MARGIN-BOTTOM: 0px"&gt;
&lt;font style="FONT-FAMILY: Times New Roman" size="2"&gt;The Company has
reviewed recent accounting pronouncements and concluded that they
are either not applicable to the business, or that no material
effect is expected on the consolidated financial statements as a
result of future adoption.&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 the basis of presentation and significant accounting policies concepts. Basis of presentation describes the underlying basis used to prepare the financial statements (for example, US Generally Accepted Accounting Principles, Other Comprehensive Basis of Accounting, IFRS).  Accounting policies describe all significant accounting policies of the reporting entity.</ElementDefenition><ElementReferences>No definition available.</ElementReferences><IsTotalLabel>false</IsTotalLabel><UnitID>0</UnitID><Label>Basis of Presentation and Summary of Significant Accounting Policies</Label></Row></Rows><Footnotes /><IsEquityReport>false</IsEquityReport><ReportName>Basis of Presentation and 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://imetrix.edgar-online.com/taxonomy/role/NotesToFinancialStatementsBasisOfPresentationAndSignificantAccountingPoliciesTextBlock</RoleURI><NumberOfCols>1</NumberOfCols><NumberOfRows>2</NumberOfRows></InstanceReport>
