A surgical stapling assembly, comprising: a staple cartridge comprising: a cartridge body comprising a lockout key; a plurality of staples stored within said staple cartridge; and a sled configured to deploy said staples during a staple firing stroke, wherein said sled is movable between an unfired position and a fired position during said staple firing stroke and wherein said staple cartridge is in an unspent configuration when said sled is in said unfired position; and a stapling instrument configured to receive said staple cartridge therein, wherein said stapling instrument comprises: a first jaw; a second jaw movable relative to said first jaw; a firing member configured to move said sled through said staple firing stroke; and a lockout comprising a lockout pocket defined in one of said first jaw and said second jaw, wherein the firing member is configured: in a first stage of the lockout, to move into said lockout pocket at the beginning of said staple firing stroke when said staple cartridge is not installed, thereby preventing distal movement of said firing member; to be lifted up by interaction between said lockout key and said firing member so that the first stage of the lockout is defeated so that the firing member does not move into said lockout pocket when said cartridge is installed, thereby allowing distal movement of said firing member; and when the first stage of the lockout has been defeated in a second stage of the lockout, to move into said lockout pocket during said staple firing stroke when said sled is not in said unfired position, thereby preventing further distal movement of said firing member; and to be lifted up by interaction between said sled and said firing member so that the second stage of the lockout is defeated so that the firing member does not move into said lockout pocket when said sled is in said unfired position, thereby allowing further distal movement of said firing member to complete said staple firing stroke. a staple cartridge comprising: a cartridge body comprising a lockout key; a plurality of staples stored within said staple cartridge; and a sled configured to deploy said staples during a staple firing stroke, wherein said sled is movable between an unfired position and a fired position during said staple firing stroke and wherein said staple cartridge is in an unspent configuration when said sled is in said unfired position; and a cartridge body comprising a lockout key; a plurality of staples stored within said staple cartridge; and a sled configured to deploy said staples during a staple firing stroke, wherein said sled is movable between an unfired position and a fired position during said staple firing stroke and wherein said staple cartridge is in an unspent configuration when said sled is in said unfired position; and a stapling instrument configured to receive said staple cartridge therein, wherein said stapling instrument comprises: a first jaw; a second jaw movable relative to said first jaw; a firing member configured to move said sled through said staple firing stroke; and a first jaw; a second jaw movable relative to said first jaw; a firing member configured to move said sled through said staple firing stroke; and a lockout comprising a lockout pocket defined in one of said first jaw and said second jaw, wherein the firing member is configured: in a first stage of the lockout, to move into said lockout pocket at the beginning of said staple firing stroke when said staple cartridge is not installed, thereby preventing distal movement of said firing member; to be lifted up by interaction between said lockout key and said firing member so that the first stage of the lockout is defeated so that the firing member does not move into said lockout pocket when said cartridge is installed, thereby allowing distal movement of said firing member; and in a first stage of the lockout, to move into said lockout pocket at the beginning of said staple firing stroke when said staple cartridge is not installed, thereby preventing distal movement of said firing member; to be lifted up by interaction between said lockout key and said firing member so that the first stage of the lockout is defeated so that the firing member does not move into said lockout pocket when said cartridge is installed, thereby allowing distal movement of said firing member; and when the first stage of the lockout has been defeated in a second stage of the lockout, to move into said lockout pocket during said staple firing stroke when said sled is not in said unfired position, thereby preventing further distal movement of said firing member; and to be lifted up by interaction between said sled and said firing member so that the second stage of the lockout is defeated so that the firing member does not move into said lockout pocket when said sled is in said unfired position, thereby allowing further distal movement of said firing member to complete said staple firing stroke. in a second stage of the lockout, to move into said lockout pocket during said staple firing stroke when said sled is not in said unfired position, thereby preventing further distal movement of said firing member; and to be lifted up by interaction between said sled and said firing member so that the second stage of the lockout is defeated so that the firing member does not move into said lockout pocket when said sled is in said unfired position, thereby allowing further distal movement of said firing member to complete said staple firing stroke.