In this paper, "reactive systems", i.e. systems which interact permanently with their environment, are considered. Such systems are encountered, for instance, in real-time control or signal processing systems, command-control-communication system, man-machine interfaces, to mention just a few. We present the Signal software environment designed and developped at INRIA-IRISA and its formal calculus system to perform the above mentioned tasks. We outline the principles of the Signal "synchronous" language and its calculus system, which is based on formal manipulations of nonlinear dynamical systems over the finite field of integers modulo 3. Then we present the prototype workstation designed at CNET, where automatic mapping of a Signal program onto a multi-transputer target architecture is performed, with the aid of a sophisticated graphic environment. Finally, we discuss what could be the impact of such tools on the future of real-time systems specification, design and implementation.