A percussion drill (10) has a tubular fluid transmitting body (14), with a drill bit (16) mounted on a drill bit support (20) itself mounted in body (14) via a spring (22) and splines (24). A mass (28) is spring mounted in the body (14) and is movable to impact on a face (26) of the support (20). Mounted to the mass (28) is a rotating valve (32) comprising two valve plates (34, 36). Each plate (34, 36) defines a slot (38, 40), such that rotation of one of the valve plates (36) relative to the other moves the slots (38, 40) into and out of alignment; the change in alignment of the slots (38, 40) alters the flow of drilling fluid through the valve (32), so altering the drilling fluid pressure force acting on the mass (28), so causing mass (28) to push against the spring (30), and impact on the drill bit support (20) providing a percussive action at the drill bit (16).