Abstract: The electrically operated disc brake (10, 110, 310, 410) may be operated by either hydraulic pressure and an electric motor or operated solely by means of an electric motor. The electric motor (40, 340, 440) drives a sun gear (52, 352, 452) of a planetary gear assembly (50, 150, 350, 450). The planetary gear assembly (50, 150, 350, 450) includes a fixed ring gear (70, 370, 470) and a rotatable ring gear (80, 180, 380, 480), the rotatable ring gear (80, 180, 380, 480) having fewer teeth than the fixed ring gear (70, 370, 470) so that rotation of the planetary gears (54, 56, 58) effects a slower rate of rotation of the rotatable ring gear (80, 180, 380, 480). The rotatable ring gear (80, 180, 380, 480) engages a screw (88, 188, 388, 488) which is connected with the piston (30, 130, 330, 430). The piston (30, 330, 430) may be displaced by hydraulic fluid for a service application and the electric motor (40, 340, 440) utilized for a parking application.