We concentrate on two major aspects of reactive system design: behavior control and modularity. These are studied from a formal point of view, within the framework of action systems. The traditional interleaving paradigm is completed with a barrier synchronization mechanism. This is achieved by introducing a new parallel composition operator, applicable to bot h discrete and hybrid models. While offering improvements with respect to control and modularity, the approach uses the correctness preserving mechanisms provided by the underlying reasoning environment 1. 1) A shorter version of this study appeared as "Modular Design of Reactive Systems", in Proceedings of the 28th Annual International Computer Software and Applications Conference ...