In event-oriented modeling, designers focus on the events that occur in time and on the causality relationship between events. This practice complements class-oriented, object-oriented, actor-oriented and state-oriented approaches. To facilitate event-oriented modeling, we have extended event graphs to create Ptera (Ptolemy event relationship actors), which we show to be appropriate for modeling complex discrete-event systems. A key capability is that Ptera models conform with an actor abstract semantics that permits hierarchical composition with other models of computation such as discrete-event actors, data ow, process networks and finite state machines. This enables their use in complex system design, where not every aspect of the system is best described with event-oriented modeling.