Apparatus for applying negative pressure therapy to a wound site, which comprises an open celled foam pad for application to the wound, a suction tube connecting the foam pad to a collection canister having a shut-off valve which closes the outlet from the canister when it is full, a tube for connecting the canister to a wall suction point or to a vacuum bottle, and a pressure detecting device connected to the suction tube between the foam pad and the canister for indicating when the pressure in the suction tube falls below a predetermined level.