Ptolemy is an environment for simulation, prototyping, and software synthesis for heterogeneous systems. It uses modern object-oriented software technology (in C++) to model each subsystem in a natural and efficient manner, and to integrate these subsystems into a whole. The objectives of Ptolemy encompass practically all aspects of designing signal processing and communications systems, ranging from algorithms and communication strategies, through simulation, hardware and software design, parallel computing, and generation of real-time prototypes. In this paper we will introduce the software synthesis aspects of the Ptolemy system. The environment presented here is both modular and extensible. Ptolemy allows the user to choose among various single- or multiple-processor schedulers.