Abstract: A clamping device, for use for example for clamping a tool or workpiece in position in a machine tool comprises a clamping anchor to which a clamping force is applied by a hydraulically operated cylinder. The device also includes a screw operated compensating or back-up unit which acts on the piston of the hydraulic cylinder. The compensating unit is driven by the same hydraulic system as the hydraulic cylinder and when the piston has been moved hydraulically to apply the clamping force to the anchor, a screw component of the compensating unit moves into engagement with the piston to lock the piston in position and maintain the clamping force even if the hydraulic pressure on the piston is then released.