Many systems are characterized by the occurrence of events, rather than the motion. Among these are elevators, for which the stopping at floors and the opening and closing of doors are of prime interest, computer systems in which the arrival and departure of data blocks are important, and process control systems for which the starting, stopping, breakdowns, and repairs are of supervisor interest. Although each of these has its internal dynamics in continuous time, witness the motion of the elevator, it is the events of stopping, etc., that of interest.