Abstract: A piston-cylinder type actuator is provided with a fail-safe unit for returning the piston 10 to a fail-safe position in the event of failure of the power supply to the cylinder 12. The fail-safe device comprises a housing unit 27 which is detachably connected to the casing 18 of the actuator. The housing contains a pre-loaded spring 30 acting between the closed end 29 of the housing and a movable cap member 31 which is engaged by the piston rod 17. The cap member is retained in the housing by a fixed ring 32 which is connected to a flange 38 of the actuator. Removal of the fail-safe unit is prevented unless the piston is in its fail-safe position by means of a bayonet-type fitting comprising lugs 35 on the housing which are axially and rotatably engaged with locking recesses on the remote surface of the adaptor flange 38.