Abstract: The invention relates to an actuator, in particular pneumatic cylinder, with an actuator element movably located in a movement space of a housing and with a microwave position sensing device for detecting the position of the actuator element in the movement space, with a microwave aerial arrangement for the transmission of microwaves into the movement space forming a waveguide and for the reception of reflection microwaves generated by the reflection of the transmitted microwaves at the actuator element from the movement space, and with evaluation means for the generation of a position signal representing the current position of the actuator element from the reflection microwaves. In the actuator, it is provided that the actuator element has a groove arrangement with at least one groove for reflecting the microwaves.