A hammer drill comprising: a body (2) in which is located a motor; a tool holder (6) capable of holding a tool bit; a hammer mechanism, driven by the motor when the motor is activated, for repetitively striking an end of the tool bit when the tool bit is held by the tool holder (6); a counter mass (20;50) slideably supported on at least one rod within the body (2) which is capable of sliding in a forward and rearward direction along a portion of the rod between two end positions; biasing means (22;62) which biases the counter mass (20;50) to a third position located between the first and second positions; wherein the counter mass is located above the centre of gravity (9) of the hammer; the mass of the counter mass (20;50) and the strength of the biasing means (22;62) being such that the counter mass (20;50) slidingly moves in forward and rearward direction to counteract vibrations generated by the operation of the hammer mechanism. The biasing means (22;62) comprises at least one helical spring which surrounds the rod and is attached to an end of the rod, wherein the at least one spring abuts the counter mass in its third position, but disengages from the counter mass as it approaches its first or second end position.