The ten meter diameter primary mirror of the W. M. Keck Telescope is a mosaic of thirty-six hexagonal mirrors. An active control system stabilizes the primary mirror. The active control system uses 168 measurements of the relative positions of adjacent mirror segments and 3 measurements of the primary mirror position in the teleL'cope structure to control the 108 degrees of freedom needed to stabilize the figure and position of the primary mirror. Th_ components of the active control system are relative position sensors, electronics, computers, actuators that position the mirrors, and software. The software algorithms control the primary mirror, perform star image stacking, emulate the segments, store and fit calibration data, and locate hardwaredefects. We give an overview of the active control system, its functional requirements and test measuremeitts. Fig. I The locations of the actuator and sensors from the concave side of the primary. '3 tj 1lr II m' _r =mi == ' .... 11' " '" " " " I?1 ' ' ....