Circular stapling instruments for cutting and applying one or more surgical staples to tissue are disclosed. The instruments include various forms of feedback systems designed to provide at least one mode of feedback to the surgeon when the knife (70) has cut through tissue clamped between a staple cartridge and an anvil portion (100). For example the knife may sever a wire (222) mounted in the anvil with the result that current flows through a circuit in parallel with the wire, thereby illuminating an indicator (250). Alternatively a magnetic sensor (260) may cause current to be supplied to an indicator when the sensor detects the presence of the knife.
