Process interaction models are an integral component of parallel computing theory and practice, defining the means by which concurrent processes interact; where "interaction" means not only the exchange of data but also synchronization between processes. In the search for a general purpose model to program parallel computers and reason about the partial orders of data in such systems, it is desirable to provide portability, an expressiveness that does not distract the programmer from the task in hand and efficiency independent of the memory architecture of the machine. This book discusses these issues and presents a new model. In this "process oriented" model a program is a collection of processes that execute concurrently, constructing and interacting via strictly typed shared data structures called "Contexts". Steven Ericsson-Zenith was a member of the Computer Architecture team at the British Semiconductor company INMOS (STMicroelectronics) during the development of the Transputer microprocessor, a device designed for large-scale parallel computing. He was also involved in the design of the parallel programming language Occam and is the author of the Occam 2 Reference Manual, published by Prentice Hall. Following his work on Occam, he was invited to continue research at YALE University with Professor David Gelernter on the coordination language Linda. This book, completed in 1992, arises from this experience and the research that followed at the Ecole Nationale Sup�rieure des Mines de Paris and the Universit� Pierre et Marie Curie, traditionally the science department of the Sorbonne.