A surgical instrument, comprising: a handle assembly; an elongated shaft assembly operably coupled to said handle assembly and protruding therefrom, said elongated shaft assembly comprising: an elongated spine member having a distal end; a firing tube axially movable on said elongated spine member and interfacing with a firing system supported by said handle assembly to cause said firing tube to selectively move on said spine member from an un-actuated position to actuated positions; and wherein said surgical instrument further comprises: a first end effector removably couplable to said distal end of said spine member, said first end effector comprising a pair of first jaws (13, 15) wherein one said first jaw is moveable relative to another said first jaw in response to said firing motions applied thereto by said firing tube when coupled to said distal end of said spine member, said surgical instrument being characterized in that it further comprises a cutting member which is movably supported within said elongated shaft assembly and which interfaces with a knife advancement system supported by said handle assembly for selectively moving said cutting member from a starting position wherein a tissue-cutting edge formed on a distal end thereof is located within a distal end of said firing tube to cutting positions wherein said tissue-cutting edge is distal to said distal end of said firing tube. a handle assembly; an elongated shaft assembly operably coupled to said handle assembly and protruding therefrom, said elongated shaft assembly comprising: an elongated spine member having a distal end; a firing tube axially movable on said elongated spine member and interfacing with a firing system supported by said handle assembly to cause said firing tube to selectively move on said spine member from an un-actuated position to actuated positions; and wherein said surgical instrument further comprises: a first end effector removably couplable to said distal end of said spine member, said first end effector comprising a pair of first jaws (13, 15) wherein one said first jaw is moveable relative to another said first jaw in response to said firing motions applied thereto by said firing tube when coupled to said distal end of said spine member, said surgical instrument being characterized in that it further comprises a cutting member which is movably supported within said elongated shaft assembly and which interfaces with a knife advancement system supported by said handle assembly for selectively moving said cutting member from a starting position wherein a tissue-cutting edge formed on a distal end thereof is located within a distal end of said firing tube to cutting positions wherein said tissue-cutting edge is distal to said distal end of said firing tube. an elongated spine member having a distal end; a firing tube axially movable on said elongated spine member and interfacing with a firing system supported by said handle assembly to cause said firing tube to selectively move on said spine member from an un-actuated position to actuated positions; and wherein said surgical instrument further comprises: a first end effector removably couplable to said distal end of said spine member, said first end effector comprising a pair of first jaws (13, 15) wherein one said first jaw is moveable relative to another said first jaw in response to said firing motions applied thereto by said firing tube when coupled to said distal end of said spine member, said surgical instrument being characterized in that it further comprises a cutting member which is movably supported within said elongated shaft assembly and which interfaces with a knife advancement system supported by said handle assembly for selectively moving said cutting member from a starting position wherein a tissue-cutting edge formed on a distal end thereof is located within a distal end of said firing tube to cutting positions wherein said tissue-cutting edge is distal to said distal end of said firing tube. a first end effector removably couplable to said distal end of said spine member, said first end effector comprising a pair of first jaws (13, 15) wherein one said first jaw is moveable relative to another said first jaw in response to said firing motions applied thereto by said firing tube when coupled to said distal end of said spine member, said surgical instrument being characterized in that it further comprises a cutting member which is movably supported within said elongated shaft assembly and which interfaces with a knife advancement system supported by said handle assembly for selectively moving said cutting member from a starting position wherein a tissue-cutting edge formed on a distal end thereof is located within a distal end of said firing tube to cutting positions wherein said tissue-cutting edge is distal to said distal end of said firing tube.