﻿<?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>BASIS OF PRESENTATION</ReportName>
  <ReportLongName>10101 - Disclosure - BASIS OF PRESENTATION</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>Unit12</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>Unit13</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>
      </Units>
      <CurrencySymbol>$</CurrencySymbol>
    </Column>
  </Columns>
  <Rows>
    <Row>
      <Id>2</Id>
      <Label>BASIS OF PRESENTATION</Label>
      <Level>0</Level>
      <ElementName>wso_BasisOfPresentationAbstract</ElementName>
      <ElementPrefix>wso</ElementPrefix>
      <IsBaseElement>false</IsBaseElement>
      <BalanceType>na</BalanceType>
      <PeriodType>duration</PeriodType>
      <ShortDefinition>Basis Of Presentation [Abstract]</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>Basis Of Presentation [Abstract]</ElementDefenition>
      <IsTotalLabel>false</IsTotalLabel>
    </Row>
    <Row>
      <Id>3</Id>
      <Label>BASIS OF PRESENTATION</Label>
      <Level>1</Level>
      <ElementName>us-gaap_OrganizationConsolidationAndPresentationOfFinancialStatementsDisclosureTextBlock</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>terselabel</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;div&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;" class="_mt" size="2"&gt;&lt;b&gt;1.&lt;/b&gt;&lt;/font&gt;&lt;/td&gt;
&lt;td valign="top" align="left"&gt;&lt;font style="font-family: Times New Roman;" class="_mt" size="2"&gt;&lt;b&gt;BASIS OF PRESENTATION &lt;/b&gt;&lt;/font&gt;&lt;/td&gt;&lt;/tr&gt;&lt;/table&gt;
&lt;p style="margin-top: 6px; margin-bottom: 0px;"&gt;&lt;font style="font-family: Times New Roman;" class="_mt" size="2"&gt;&lt;b&gt;Basis of Consolidation &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;" class="_mt" size="2"&gt;Watsco, Inc. and its subsidiaries (collectively, "Watsco," which may be referred to as &lt;i&gt;we, us &lt;/i&gt;or &lt;i&gt;our&lt;/i&gt;) was incorporated in 1956 and is the largest distributor of air conditioning, heating and refrigeration equipment and related parts and supplies ("HVAC/R") in the HVAC/R distribution industry. The accompanying Watsco September&amp;nbsp;30, 2010 interim condensed consolidated unaudited financial statements have been prepared pursuant to the rules and regulations of the Securities and Exchange Commission. Certain information and note disclosures normally included in the annual financial statements prepared in accordance with U.S. generally accepted accounting principles ("GAAP") have been condensed or omitted pursuant to those rules and regulations, although we believe the disclosures made are adequate to make the information presented not misleading. In the opinion of management, all adjustments, consisting of normal and recurring adjustments, necessary for a fair presentation have been included in the condensed consolidated unaudited financial statements herein. These statements should be read in conjunction with the consolidated financial statements and notes thereto included in our 2009 Annual Report on Form 10-K. &lt;/font&gt;&lt;/p&gt;
&lt;p style="margin-top: 12px; margin-bottom: 0px;"&gt;&lt;font style="font-family: Times New Roman;" class="_mt" size="2"&gt;The condensed consolidated unaudited financial statements contained in this report include the accounts of Watsco and all of its wholly-owned subsidiaries and include the accounts of a joint venture in which Watsco maintains a 60% controlling interest. All significant intercompany balances and transactions have been eliminated. &lt;/font&gt;&lt;/p&gt;
&lt;p style="margin-top: 12px; margin-bottom: 0px;"&gt;&lt;font style="font-family: Times New Roman;" class="_mt" size="2"&gt;The results of operations for the quarter and nine months ended September&amp;nbsp;30, 2010 are not necessarily indicative of the results to be expected for the year ending December&amp;nbsp;31, 2010. Sales of residential central air conditioners, heating equipment and parts and supplies are seasonal. Furthermore, results of operations can be impacted favorably or unfavorably based on weather patterns during the summer and winter selling seasons. Demand related to the residential central air conditioning replacement market is highest in the second and third quarters, and demand for heating equipment is usually highest in the fourth quarter. Demand related to the new construction sectors throughout most of the markets is fairly consistent during the year except for dependence on housing completions and related weather and economic conditions. &lt;/font&gt;&lt;/p&gt;
&lt;p style="margin-top: 18px; margin-bottom: 0px;"&gt;&lt;font style="font-family: Times New Roman;" class="_mt" size="2"&gt;&lt;b&gt;Use of Estimates &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;" class="_mt" size="2"&gt;The preparation of condensed consolidated unaudited financial statements in conformity with GAAP requires 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 condensed consolidated unaudited financial statements and the reported amounts of revenues and expenses for the reporting period. Significant estimates include valuation reserves for accounts receivable, inventory and income taxes, reserves related to self-insurance programs and valuation of goodwill and indefinite lived intangible assets. While we believe that these estimates are reasonable, actual results could differ from such estimates. &lt;/font&gt;&lt;/p&gt;
&lt;p style="margin-top: 18px; margin-bottom: 0px;"&gt;&lt;font style="font-family: Times New Roman;" class="_mt" size="2"&gt;&lt;b&gt;Recently Adopted Accounting Standards &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;" class="_mt" size="2"&gt;&lt;i&gt;Transfers of Financial Assets &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;" class="_mt" size="2"&gt;On January&amp;nbsp;1, 2010, we adopted a new accounting standard issued by the Financial Accounting Standards Board ("FASB") that addresses the accounting and disclosure requirements for transfers of financial assets. The adoption of this guidance did not have a material impact on our condensed consolidated unaudited financial statements. &lt;/font&gt;&lt;/p&gt;
&lt;p style="margin-top: 18px; margin-bottom: 0px;"&gt;&lt;font style="font-family: Times New Roman;" class="_mt" 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;" class="_mt" size="2"&gt;On January&amp;nbsp;1, 2010, we adopted accounting guidance issued by the FASB that amended the consolidation principles for variable interest entities ("VIEs") by requiring consolidation of VIEs based on whether an entity has the power to direct the activities that most significantly impact the VIEs economic performance and shares in the significant risks and rewards of the entity. The adoption of this guidance did not have a material impact on our condensed consolidated unaudited financial statements. &lt;/font&gt;&lt;/p&gt;
&lt;p style="margin-top: 18px; margin-bottom: 0px;"&gt;&lt;font style="font-family: Times New Roman;" class="_mt" size="2"&gt;&lt;b&gt;Recently Issued Accounting Standards &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;" class="_mt" size="2"&gt;&lt;i&gt;Amendments to Revenue Arrangements with Multiple Deliverables &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;" class="_mt" size="2"&gt;In October 2009, the FASB issued accounting guidance that amends the criteria for allocating a contract's consideration to individual services or products in multiple deliverable arrangements. This guidance is effective prospectively for revenue arrangements entered into or materially modified in fiscal years beginning on or after June&amp;nbsp;15, 2010. We will adopt this guidance beginning on January&amp;nbsp;1, 2011 and do not expect a material impact on our consolidated financial statements. &lt;/font&gt;&lt;/p&gt; &lt;/div&gt;</NonNumbericText>
          <NonNumericTextHeader>1.
BASIS OF PRESENTATION
Basis of Consolidation
Watsco, Inc. and its subsidiaries (collectively, "Watsco," which may be referred to as we, us or our) was</NonNumericTextHeader>
          <FootnoteIndexer />
          <hasSegments>false</hasSegments>
          <hasScenarios>false</hasScenarios>
          <DisplayDateInUSFormat>false</DisplayDateInUSFormat>
        </Cell>
      </Cells>
      <OriginalInstanceReportColumns />
      <ElementDataType>us-types:textBlockItemType</ElementDataType>
      <SimpleDataType>textblock</SimpleDataType>
      <ElementDefenition>Description containing the entire organization, consolidation and basis of presentation of financial statements disclosure. May be provided in more than one note to the financial statements, as long as users are provided with an understanding of (1) the significant judgments and assumptions made by an enterprise in determining whether it must consolidate a VIE and/or disclose information about its involvement with a VIE, (2) the nature of restrictions on a consolidated VIE's assets reported by an enterprise in its statement of financial position, including the carrying amounts of such assets, (3) the nature of, and changes in, the risks associated with an enterprise's involvement with the VIE, and (4) how an enterprise's involvement with the VIE affects the enterprise's financial position, financial performance, and cash flows.  Describes procedure if disclosures are provided in more than one note to the financial statements.</ElementDefenition>
      <ElementReferences>Reference 1: http://www.xbrl.org/2003/role/presentationRef
 -Publisher FASB
 -Name FASB Staff Position (FSP)
 -Number FAS140-4 and FIN46(R)-8
 -Paragraph 8, C1, C7

Reference 2: http://www.xbrl.org/2003/role/presentationRef
 -Publisher AICPA
 -Name Accounting Research Bulletin (ARB)
 -Number 51
 -Paragraph 2-6

Reference 3: http://www.xbrl.org/2003/role/presentationRef
 -Publisher AICPA
 -Name Statement of Position (SOP)
 -Number 94-6
 -Paragraph 10

Reference 4: http://www.xbrl.org/2003/role/presentationRef
 -Publisher FASB
 -Name FASB Interpretation (FIN)
 -Number 46R
 -Paragraph 4, 14, 15

</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>
