The Ptolemy II Discrete Event (DE) domain provides a general environment for time-oriented simulations of systems such as queueing systems, communication networks, and hardware systems. In this domain, actors communicate by sending tokens across connections. The token sent and the time at which the action took place constitute an event in the DE domain. Upon receiving an event, the destination actor is activated and a reaction take place. The reaction may change the internal state of the actor and possibly generate new events, resulting in further reactions. A DE domain scheduler ensures that events are processed chronologically.