A recoilless hammer comprising a moil supported for reciprocal movement along a hammer axis of a housing, a piston moveable within the chamber of an elongate tube extending from the rear of the housing such that it may strike the moil. In a rest position the piston is held forward against the moil by a low pressure air supply delivered from the aft end of the chamber, and the piston is retracted to a charged position at the aft end of the chamber by delivery of high pressure air to act on the fore portion of the piston. Upon actuation of a trigger mechanism, air is vented from the fore end of the chamber to atmosphere causing a pressure unbalance of low magnitude across the piston, such that it accelerates towards and strikes the moil. The chamber is of a length to enable a high energy blow with minimal piston acceleration recoil when the hammer is manually held and operated.