A surgical cutting and fastening instrument comprising: a handle assembly; a generator of rotary motion supported by said handle assembly; an elongate shaft assembly operably coupled to said handle assembly and defining an elongate axis; an elongate channel coupled to said elongate shaft assembly for selective pivotal travel about a pivot axis that is substantially transverse to said elongate axis, said elongate channel sized to operably support a staple cartridge therein; and a rotary drive system comprising: a main drive shaft assembly operably interfacing with said generator of rotary motion; a gear drive train (50, 52, 54) in meshing engagement with said main drive shaft for receiving said rotary motion therefrom; and a drive shaft in meshing engagement with said gear drive train, said drive shaft further having a distal end coupled to a knife supporting assembly in said elongate channel to impart an axial pushing motion thereto, to drive said knife supporting assembly in a distal direction upon receipt of a rotary motion in a first direction from said gear drive train (50, 52, 54); wherein said distal end of said drive shaft is non-threadably coupled to said knife supporting assembly such that upon receipt of another rotary motion in a second direction from said gear drive train (54, 56), said drive shaft pulls said knife supporting assembly in a proximal direction, characterized in that: upon application of said another rotary motion in said second direction to said drive shaft, said drive shaft moves axially in a proximal direction. a handle assembly; a generator of rotary motion supported by said handle assembly; an elongate shaft assembly operably coupled to said handle assembly and defining an elongate axis; an elongate channel coupled to said elongate shaft assembly for selective pivotal travel about a pivot axis that is substantially transverse to said elongate axis, said elongate channel sized to operably support a staple cartridge therein; and a rotary drive system comprising: a main drive shaft assembly operably interfacing with said generator of rotary motion; a gear drive train (50, 52, 54) in meshing engagement with said main drive shaft for receiving said rotary motion therefrom; and a drive shaft in meshing engagement with said gear drive train, said drive shaft further having a distal end coupled to a knife supporting assembly in said elongate channel to impart an axial pushing motion thereto, to drive said knife supporting assembly in a distal direction upon receipt of a rotary motion in a first direction from said gear drive train (50, 52, 54); wherein said distal end of said drive shaft is non-threadably coupled to said knife supporting assembly such that upon receipt of another rotary motion in a second direction from said gear drive train (54, 56), said drive shaft pulls said knife supporting assembly in a proximal direction, characterized in that: upon application of said another rotary motion in said second direction to said drive shaft, said drive shaft moves axially in a proximal direction. a main drive shaft assembly operably interfacing with said generator of rotary motion; a gear drive train (50, 52, 54) in meshing engagement with said main drive shaft for receiving said rotary motion therefrom; and a drive shaft in meshing engagement with said gear drive train, said drive shaft further having a distal end coupled to a knife supporting assembly in said elongate channel to impart an axial pushing motion thereto, to drive said knife supporting assembly in a distal direction upon receipt of a rotary motion in a first direction from said gear drive train (50, 52, 54); wherein said distal end of said drive shaft is non-threadably coupled to said knife supporting assembly such that upon receipt of another rotary motion in a second direction from said gear drive train (54, 56), said drive shaft pulls said knife supporting assembly in a proximal direction, characterized in that: upon application of said another rotary motion in said second direction to said drive shaft, said drive shaft moves axially in a proximal direction.