[en] Dual computer control systems are used for direct digital control of reactivity and other major process parameters in 27 full size commercial CANDU nuclear units, 16 of which are in-service. The control computers are also used for supervisory control, alarm annunciation on CRT displays, and graphic data presentation on CRT displays. The first dual computer control system for a commercial CANDU unit, was commissioned on Pickering A unit 1, which was declared in-service in 1971. The control functions, man-machine interface, and fault tolerant features of the dual computer control systems for the current CANDU units are described. The development of a distributed control system concept for future CANDU plants incorporating microprocessor implementation of logic functions, remote multiplexing, and redundant data highways, is also described