The previous chapters have modeled computer programs mainly as “black boxes” that accept some input and produce some output; the internal operation of a program to achieve this behavior was considered as “irrelevant” to the external observer and thus deliberately hidden. While this view is indeed adequate to model sequential program executions, it fails if we wish to consider concurrent systems, i.e., systems where multiple components (cores, processors, computers) execute multiple activities (threads, processes) in parallel. The various components may interact with each other by synchronization respectively communication; such systems of components that react to influences from other components are also called reactive systems.