The description relates to an actuator for use at high and low temperatures, with a cylinder block (8), a piston (14) movable therein and a piston rod (22) secured to the piston. A first and a second flange cover (46, 48) and a sleeve (40) in the cylinder block (8) in which the piston (14) can move to and fro are made of ceramic. The piston rod (22) is secured to the piston (14), also of ceramic, by a head bearing (24) movable transversely to the longitudinal axis of the piston. The seal between the piston (14) and the sleeve (40) is provided by a close fit alone, without the need for additional sealing components. The low heat expansion of the ceramic components used also means that the tolerances change little even at great temperature fluctuations. The radial head bearing (24) prevents the piston (14) from jamming during forces transverse to the longitudinal axis of the piston possibly arising at the piston rod (22).