The authors describe LUSTRE, a data flow synchronous language designed for programming reactive systems-such as automatic control and monitoring systems-as well as for describing hardware. The data flow aspect of LUSTRE makes it very close to usual description tools in these domains (block-diagrams, networks of operators, dynamical sample-systems, etc.), and its synchronous interpretation makes it well suited for handling time in programs. Moreover, this synchronous interpretation allows it to be compiled into an efficient sequential program. The LUSTRE formalism is very similar to temporal logics. This allows the language to be used for both writing programs and expressing program properties, which results in an original program verification methodology.< >