A surgical stapling instrument, comprising: a handle assembly; a shaft assembly coupled to said handle assembly; a stapling head assembly operably coupled to said shaft assembly, said stapling head assembly comprising: a staple cartridge for supporting one or more surgical staples; a staple driver for engaging and driving the staples from said staple cartridge; and a knife movably supported in said stapling head assembly; a drive system for applying drive motions to said staple driver and said knife; an anvil movably supported relative to said staple cartridge for axial movement toward and away from said staple cartridge; an anvil adjustment assembly configured to selectively adjust an axial position of the anvil relative to the staple cartridge, said the anvil adjustment assembly comprising: an adjustment shaft movably supported by said handle assembly for selective axial travel therethrough; and a trocar portion coupled to said adjustment shaft for axial travel therewith, said trocar portion configured for removable attachment to said anvil; wherein the surgical stapling instrument further comprises: a force measurement system operably interfacing with said anvil adjustment assembly for measuring an amount of compression force applied to tissue clamped between said anvil and said staple cartridge; wherein the surgical stapling instrument is characterised in that it further comprises an anvil locking system configured to cooperate with said anvil adjustment assembly to selectively lock and unlock the anvil in the axial position relative to said staple cartridge, said anvil locking system comprising: a first locking member comprising an elongated gear rack, the first locking member coupled to one of said adjustment shaft and said trocar portion; and a second locking member comprising a locking assembly, the locking assembly pivotally coupled to said handle assembly and configured to pivot between a locked position wherein said locking assembly retainingly engages said first elongated gear rack to prevent axial travel thereof and an unlocked position wherein said locking assembly does not engage said elongated gear rack. a handle assembly; a shaft assembly coupled to said handle assembly; a stapling head assembly operably coupled to said shaft assembly, said stapling head assembly comprising: a staple cartridge for supporting one or more surgical staples; a staple driver for engaging and driving the staples from said staple cartridge; and a knife movably supported in said stapling head assembly; a staple cartridge for supporting one or more surgical staples; a staple driver for engaging and driving the staples from said staple cartridge; and a knife movably supported in said stapling head assembly; a drive system for applying drive motions to said staple driver and said knife; an anvil movably supported relative to said staple cartridge for axial movement toward and away from said staple cartridge; an anvil adjustment assembly configured to selectively adjust an axial position of the anvil relative to the staple cartridge, said the anvil adjustment assembly comprising: an adjustment shaft movably supported by said handle assembly for selective axial travel therethrough; and a trocar portion coupled to said adjustment shaft for axial travel therewith, said trocar portion configured for removable attachment to said anvil; an adjustment shaft movably supported by said handle assembly for selective axial travel therethrough; and a trocar portion coupled to said adjustment shaft for axial travel therewith, said trocar portion configured for removable attachment to said anvil; a force measurement system operably interfacing with said anvil adjustment assembly for measuring an amount of compression force applied to tissue clamped between said anvil and said staple cartridge; wherein the surgical stapling instrument is characterised in that it further comprises an anvil locking system configured to cooperate with said anvil adjustment assembly to selectively lock and unlock the anvil in the axial position relative to said staple cartridge, said anvil locking system comprising: a first locking member comprising an elongated gear rack, the first locking member coupled to one of said adjustment shaft and said trocar portion; and a second locking member comprising a locking assembly, the locking assembly pivotally coupled to said handle assembly and configured to pivot between a locked position wherein said locking assembly retainingly engages said first elongated gear rack to prevent axial travel thereof and an unlocked position wherein said locking assembly does not engage said elongated gear rack. a first locking member comprising an elongated gear rack, the first locking member coupled to one of said adjustment shaft and said trocar portion; and a second locking member comprising a locking assembly, the locking assembly pivotally coupled to said handle assembly and configured to pivot between a locked position wherein said locking assembly retainingly engages said first elongated gear rack to prevent axial travel thereof and an unlocked position wherein said locking assembly does not engage said elongated gear rack.