The PVM system, a software framework for heterogeneous concurrent computing in networked environments, has evolved in the past several years into a viable technology for distributed and parallel processing in a variety of disciplines. PVM supports a straightforward but functionally complete message passing model, and is capable of harnessing the combined resources of typically heterogeneous networked computing platforms to deliver high levels of performance and functionality. In this paper, we describe the architecture of PVM system, and discuss its computing model, the programming interface it supports, auxiliary facilities for process groups and MPP support, and some of the internal implementation techniques employed. Performance issues, dealing primarily with communication overheads, are analyzed, and recent findings as well as experimental enhancements to are presented. In order to demonstrate the viability of PVM for large scale scientific supercomputing, the paper incl...