The ACIS instrument has been operating for three years in orbit, producing high quality scientific data on a wide variety of X-ray emitting astronomical objects. Except for a brief period at the very beginning of the mission when the CCDs were exposed to the radiation environment of the Outer van Allen Belts which resulted in substantial radiation damage to the front illuminated CCDs, the instrument has operated nearly flawlessly. The following report presents a description of the instrument, the current status of the instrument calibration and a few highlights of the scientific results obtained from the Guaranteed Observer Time.