A surgical instrument, comprising: a cartridge supporting assembly configured to operably support a staple cartridge therein; a firing member operably supported relative to said cartridge supporting assembly and being selectively translatable from an unfired position to a fired position in a staple firing motion in response to a firing force applied to said firing member and to retract to an unfired position in response to a retraction force applied to said firing member, said firing member having a vertical portion and a pair of upper protrusions protruding therefrom; an anvil comprising an anvil body pivotally coupled to said cartridge supporting assembly and being selectively pivotable between an open position and a closed position in response to opening and closing motions respectively applied thereto and having a first anvil ledge formed therein and a second anvil ledge formed therein and spaced apart from said first anvil ledge to define an elongate anvil slot therebetween through which said vertical portion of said firing member may pass when said anvil is in said closed position such that said upper protrusions coact with said first anvil ledge and said second anvil ledge, and said anvil having a staple forming undersurface therein and characterised in that said staple forming undersurface comprises: a plurality of right inserts (310, 320, 330, 340, 380, 400) movably supported by said anvil body on a first side of said anvil slot and having a first staple forming undersurface thereon; and a right compliant member (350, 412, 430) supported between each right insert (310, 320, 330, 340, 380, 400) and said anvil body; a plurality of left inserts (270, 280, 290, 300, 380, 400) movably supported by said anvil body on a left side of said anvil slot and having a second staple forming undersurface thereon; and a left compliant member (350, 412, 420) supported between each said left insert (270, 280, 290, 300, 380, 400) and said anvil body; and wherein each of the plurality of right inserts and the plurality of left inserts of said anvil are movable relative to other portions of said anvil to allow a distance between the staple forming undersurface of said anvil and the upper surface of a staple cartridge supported within said cartridge supporting assembly to vary in relation to a thickness of tissue clamped therebetween. a cartridge supporting assembly configured to operably support a staple cartridge therein; a firing member operably supported relative to said cartridge supporting assembly and being selectively translatable from an unfired position to a fired position in a staple firing motion in response to a firing force applied to said firing member and to retract to an unfired position in response to a retraction force applied to said firing member, said firing member having a vertical portion and a pair of upper protrusions protruding therefrom; an anvil comprising an anvil body pivotally coupled to said cartridge supporting assembly and being selectively pivotable between an open position and a closed position in response to opening and closing motions respectively applied thereto and having a first anvil ledge formed therein and a second anvil ledge formed therein and spaced apart from said first anvil ledge to define an elongate anvil slot therebetween through which said vertical portion of said firing member may pass when said anvil is in said closed position such that said upper protrusions coact with said first anvil ledge and said second anvil ledge, and said anvil having a staple forming undersurface therein and characterised in that said staple forming undersurface comprises: a plurality of right inserts (310, 320, 330, 340, 380, 400) movably supported by said anvil body on a first side of said anvil slot and having a first staple forming undersurface thereon; and a right compliant member (350, 412, 430) supported between each right insert (310, 320, 330, 340, 380, 400) and said anvil body; a plurality of left inserts (270, 280, 290, 300, 380, 400) movably supported by said anvil body on a left side of said anvil slot and having a second staple forming undersurface thereon; and a left compliant member (350, 412, 420) supported between each said left insert (270, 280, 290, 300, 380, 400) and said anvil body; and a plurality of right inserts (310, 320, 330, 340, 380, 400) movably supported by said anvil body on a first side of said anvil slot and having a first staple forming undersurface thereon; and a right compliant member (350, 412, 430) supported between each right insert (310, 320, 330, 340, 380, 400) and said anvil body; a plurality of left inserts (270, 280, 290, 300, 380, 400) movably supported by said anvil body on a left side of said anvil slot and having a second staple forming undersurface thereon; and a left compliant member (350, 412, 420) supported between each said left insert (270, 280, 290, 300, 380, 400) and said anvil body; and wherein each of the plurality of right inserts and the plurality of left inserts of said anvil are movable relative to other portions of said anvil to allow a distance between the staple forming undersurface of said anvil and the upper surface of a staple cartridge supported within said cartridge supporting assembly to vary in relation to a thickness of tissue clamped therebetween.