State Transition Diagrams

State transition diagrams are a graphic notation that has long been used to represent computing systems. Two basic models of state transition diagrams were introduced simultaneously by G.H. Mealy and E.F. Moore in the mid fifties, and have played a major role in hardware design for a long time. These basic models have been expanded significantly in the recent past to include such features as the ability to represent hierarchy, timing and communication, and have been used to model complex software systems. In this contribution, we discuss the original models of state transition diagrams, their semantic definition and their extensions; then we discuss current application domains and support tools.