A clutch assembly for a surface treatment appliance includes an input clutch member, an output clutch member driven by the input clutch member, a control system for determining the difference between the rotational speeds of the input clutch member and the output clutch member, and a drive system for effecting relative movement between the input clutch member and the output clutch member from an engaged position, in which torque is transmitted by the output clutch member to an output, to a disengaged position, in which torque is not transmitted to the output, depending on said difference between the rotational speeds.