Abstract: An automatic sliding door system suited for use in walk-in freezer or refrigerator units includes exterior and interior sensor units for opening one or more sliding doors. Each sensor unit comprises both a motion sensor and a presence detector. The motion sensor detects motion towards the sliding door(s), and opens the door(s) as they are approached. The motion sensor keeps the door(s) open so long as the motion is present. The presence detector of each sensor unit detects a change in the ambient conditions directly underneath the doorway, and keeps the door(s) open when the ambient conditions change. A brushless DC servo motor is preferably employed to move a pulley and thereby synchronously open and close the door(s). Optical sensors are deployed above the door(s) at selected deceleration and stopping points for the doors. Other features include a push plate or pushbutton, a key lock, and a multi-tier alarm system.