The concept of “process ” has come to play a central role in many efforts to master the complexity of large computer systems. The purpose of this paper is to discuss useful methods of structuring complex processes, and to relate these to the problems of improving the quality of large computer systems. Two distinct ways of structuring systems are presented, namely, process combination, and process abstraction; these are then used to discuss such topics as concurrency, synchronization, multiprogramming, interpreters, and programmable processors. This discussion is based on a set of precise definitions for such concepts as “process,” “processor, ” “computation, ” “combination, ” and “abstraction. ” The paper relates these definitions to both current research and practical applications, with particular concern for the problems of the performance, reliability, and modifiability of computer systems. Key words and phrases: sequential process, cooperating processes, asynchronous processes, parallelism, complexity, program, processor, interpreter, hierarchical structures, abstraction, refinement.