Much has been learned in the last decade about concurrent programming. This paper identifies the major concepts and describes some of the more important language notations for writing concurrent programs. The roles of processes, communication and syhchronization are discussed from both an operational and an axiomatic viewpoint. Language notations for expressing concurrent execution and for specifying process interaction are surveyed. Synchronization primitives based on shared variables and on message passing are described. Finally, three general classes of concurrent programming languages are identified and compared.