A logic system that operates with synchronous timing, i.e. the timing of all the switching operations is controlled by clock pulses. A logic system is said to be asynchronous when all the switching operations are triggered by a free-running signal so that successive stages or instructions are triggered by the completion of operation of the preceding stage.

Synchronous logic in general is slower and the timing more critical than asynchronous logic, but usually fewer and simpler circuits are required.