Thoughitstitle"IntegralGeometry"mayappearsomewhatunusualinthiscontextitisneverthelessquiteappropriate,forIntegralGeometryisanoutgrowthofwhatintheoldendayswasreferredtoas"geometricprobabil-ities." Originating,aslegendhasit,withtheBuffonneedleproblem(whichafternearlytwocenturieshaslostlittleofitseleganceandappeal),geometricprobabilitieshaverunintodifficultiesculminatingintheparadoxesofBertrandwhichthreatenedthefledglingfieldwithbanishmentfromthehomeofMathematics.Inrescuingitfromthisfate,Poincar6madethesuggestionthatthearbitrarinessofdefinitionunderlyingtheparadoxescouldberemovedbytyingcloserthedefinitionofprobabilitywithageometricgroupofwhichitwouldhavetobeaninvariant.