The book provides a detailed description of a set of languages for modeling reactive systems, which underlies the STATEMATE toolset. The approach is dominated by the language of Statecharts, used to describe behavior, combined Activity-charts for describing activities (i.e., the functional building blocks--capabilities or objects) and the data that flows between them. These two languages are used to develop a conceptual model of the system, which can be combined with the system's physical, or structural model, described in a third language--Module-charts. The three languages are highly diagrammatic in nature, constituting full-fledged visual formalisms, complete with rigorous semantics. They are accompanied by a Data Dictionary for specifying additional parts of the model that are textual in nature.