This volume gives the proceedings of the conference CONCUR '90. This is the first conference organized by ESPRIT Basic Research Action 3006, CONCUR (Theories of Concurrency: Unification and Extension), started in September 1989. The principal aims of the action are to explore the relationships among the different approaches to algebraic concurrency theory, and to develop a formalism applicable to a wide range of case studies. The articles in this volume describe the state of the art in concurrency, the theory of communicating concurrent (or distributed) systems.