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 (COMPSAC 2004), IEEE Computer Society Press, September 2004, Hong Kong. Pages 265-271.