An actuator assembly (1) comprises a body (2) in which works an actuating piston (7), a first piston (5) and at least one second piston (6), a chamber (26) containing a substantially incompressible fluid by which each of the first and second pistons (5, 6) acts on the actuating piston (7). The movement of the first piston (5) from a retracted position to an extended position acts via the fluid to cause the actuating piston (7) to move from a retracted position to an operational position, and subsequent movement of a second piston (6) from a retracted position to an extended position acts via the fluid to cause an actuation movement of the actuating piston (7).