A surgical instrument comprising: a handle assembly including a stationary handle and a movable handle; an elongate body (12, 16) extending distally from the handle assembly and defining a longitudinal axis, the elongate body (12, 16) having a distal portion comprising an end effector, said end effector having a first jaw member supported by the elongate body (12, 16) and containing a plurality of fasteners and a second jaw member mounted adjacent the first jaw member, the second jaw member being configured to move in relation to the first jaw member between an open position and a closed position upon actuation of the handle assembly; and an actuation shaft and a drive rod coupling the movable handle to the end effector; characterised in that: the actuation shaft has a first set of teeth pointing distally along the longitudinal axis and a second set of teeth pointing proximally along the longitudinal axis, and the surgical instrument further comprises a return pawl and an advancement pawl pivotably connected to the moveable handle, the return pawl being biased to pivot about a distal region of the return pawl to engage the first set of teeth, and the advancement pawl being biased to pivot about a proximal region of the advancement pawl to engage the second set of teeth, a slider cam (62, 262) selectively moveable between a first position in which it is between the return pawl and the shaft to disengage the return pawl from the first set of teeth and a second position in which it is between the advancement pawl and the shaft to disengage the advancement pawl from the second set of teeth, and a switch (60; 260) operatively connected to the slider cam (62, 262), the switch (60; 260) being configured to move the slider cam between the first and second positions such that, in the first position actuation of the movable handle in a first direction advances the actuation shaft and the drive rod distally, and such that, in the second position actuation of the movable handle in the first direction retracts the actuation shaft and the drive rod proximally. a handle assembly including a stationary handle and a movable handle; an elongate body (12, 16) extending distally from the handle assembly and defining a longitudinal axis, the elongate body (12, 16) having a distal portion comprising an end effector, said end effector having a first jaw member supported by the elongate body (12, 16) and containing a plurality of fasteners and a second jaw member mounted adjacent the first jaw member, the second jaw member being configured to move in relation to the first jaw member between an open position and a closed position upon actuation of the handle assembly; and an actuation shaft and a drive rod coupling the movable handle to the end effector; characterised in that: the actuation shaft has a first set of teeth pointing distally along the longitudinal axis and a second set of teeth pointing proximally along the longitudinal axis, and the surgical instrument further comprises a return pawl and an advancement pawl pivotably connected to the moveable handle, the return pawl being biased to pivot about a distal region of the return pawl to engage the first set of teeth, and the advancement pawl being biased to pivot about a proximal region of the advancement pawl to engage the second set of teeth, a slider cam (62, 262) selectively moveable between a first position in which it is between the return pawl and the shaft to disengage the return pawl from the first set of teeth and a second position in which it is between the advancement pawl and the shaft to disengage the advancement pawl from the second set of teeth, and a switch (60; 260) operatively connected to the slider cam (62, 262), the switch (60; 260) being configured to move the slider cam between the first and second positions such that, in the first position actuation of the movable handle in a first direction advances the actuation shaft and the drive rod distally, and such that, in the second position actuation of the movable handle in the first direction retracts the actuation shaft and the drive rod proximally. the actuation shaft has a first set of teeth pointing distally along the longitudinal axis and a second set of teeth pointing proximally along the longitudinal axis, and the surgical instrument further comprises a return pawl and an advancement pawl pivotably connected to the moveable handle, the return pawl being biased to pivot about a distal region of the return pawl to engage the first set of teeth, and the advancement pawl being biased to pivot about a proximal region of the advancement pawl to engage the second set of teeth, a slider cam (62, 262) selectively moveable between a first position in which it is between the return pawl and the shaft to disengage the return pawl from the first set of teeth and a second position in which it is between the advancement pawl and the shaft to disengage the advancement pawl from the second set of teeth, and a switch (60; 260) operatively connected to the slider cam (62, 262), the switch (60; 260) being configured to move the slider cam between the first and second positions such that, in the first position actuation of the movable handle in a first direction advances the actuation shaft and the drive rod distally, and such that, in the second position actuation of the movable handle in the first direction retracts the actuation shaft and the drive rod proximally.