this paper, we shall mostly be concerned by reactive kernels that constitute the central and most difficult part of reactive systems. In fact, ESTEREL is not a full-fledged programming language, but rather a program generator used to program reactive kernels in the same way as YACC [32] is used to program parsers from grammars. The interface and data handling must be specified in some host language. 1.2. Deterministic reactive programs Determinism is an important characteristic of reactive programs. A deterministic reactive program produces identical output sequences when fed with identical input sequences. All examples above are deterministic if physical time is considered as an input among others. The importance of determinism cannot be overestimated: deterministic systems are one order of magnitude simpler to specify, debug, and analyze than non-deterministic ones