Finite state machines have a fundamental importance for realtime software development. A comparison of the Mealy and Moore models for finite state machines indicates certain advantages of the Moore model over the almost industry-standard Mealy model. A refinement of the Moore model is proposed that distinguishes between continuous activities taken while the system is in a specific state, and those required at the transitions into and out of the state. Methods for using FSMs in the design model of a real-time system are discussed.