An automatic or self-winding watch is a mechanical wrist watch in which the mainspring is wound automatically as a result of natural motion of the wearer's arm, to provide energy to run the watch, making manual winding unnecessary. A mechanical watch which is neither self-winding nor electrically driven may be called a manual watch. Many of the mechanical watches sold today are self-winding. Others require manual winding on a daily or weekly basis; still others use mechanical or solar energy to charge a battery driving a quartz movement.

A mechanical watch is powered by an internal spiral mainspring which turns the gears that move the hands. The spring loses energy as the watch runs, so in a manual watch movement the spring must be wound by turning a small knob on the case to provide energy to run the watch otherwise, once the watch loses its stored energy, it stops.

A self-winding watch movement has a mechanism which winds the mainspring. The watch contains an eccentric weight (the rotor), which turns on a pivot. The normal movements of the user's arm cause the rotor to pivot on its staff, which is attached to a ratcheted winding mechanism. The motion of the wearer's arm is thereby translated into circular motion of the rotor which, through a series of reverser and reducing gears, eventually winds the mainspring. Modern self-winding mechanisms have two ratchets and wind the mainspring during clockwise and anticlockwise rotor motions.