Abstract: A linear actuator comprises rotary motion producing means (1,2,3,4,5); linear motion producing means (6,7,10,11,12) coupled with the rotary motion producing means for converting rotary motion to linear motion; a driven member (18) movable linearly by the linear motion providing means from a first position to a second position; and backdrive means (22) for returning the driven member to the first position. The arrangement is such that the linear motion producing means includes torque reaction means (13,15,16,17) which, in normal operation, is in an activated condition and provides a torque reaction path to enable the driven member to be moved from the first to the second position but which, in the event of a fault, is in a de-activated condition so that it no longer provides the torque reaction path and the back-drive means can move the driven member to the first position, without disengaging the rotary motion producing means from the linear motion producing means.