: Active messages provide an important new communication primitive for building message passing systems. CMMD, the message passing system of the CM-5, uses active messages as a basic substrate for constructing multiple, low overhead, communication paradigms. Examples are also given which show how developers may incorporate active messages in application-specific ways. KEYWORDS: Message passing, active messages, massively parallel systems 1. Introduction The current generation of distributed-memory massively parallel systems stand in marked contrast to systems of only a few years ago. Hardware advancements have brought improved floating point performance and communication rates. Node-to-node network latencies are now in the microsecond range and bandwidths range from the tens to hundreds of megabytes per second. Message passing libraries and compilers for parallel languages alike benefit from improved communication performance. However, as network latencies decrease and bandwidths inc..