Partial order semantics of concurrent programs -- SCONE: A simple calculus of nets -- Value-passing in process algebras -- Let's make models -- Ideal specification formalism = expressivity + compositionality + decidability + testability + ... -- An axiomatization of Lamport's temporal logic of actions -- Convergence of iteration systems -- Process algebra with a zero object -- On the asynchronous nature of communication in concurrent logic languages: A fully abstract model based on sequences -- Verifying temporal properties of processes -- Testing equivalences and fully abstract models for probabilistic processes -- A preorder for partial process specifications -- Back and forth bisimulations -- Reduction and design of well-behaved concurrent systems -- Synthesis rules for Petri nets, and how they lead to new results -- The need for headers: An impossibility result for communication over unreliable channels -- A temporal approach to algebraic specifications -- Superimposition for interacting processes -- An implementation of a translational semantics for an imperative language -- CCSR: A calculus for communicating shared resources -- The linear time - branching time spectrum -- A programming logic for formal concurrent systems -- A new strategy for proving ?-completeness applied to process algebra -- Transition system specifications with negative premises -- Delay-insensitive circuits: An algebraic approach to their design -- Equivalences, congruences, and complete axiomatizations for probabilistic processes -- Rewriting as a unified model of concurrency -- A temporal calculus of communicating systems -- Proving termination of communicating programs -- Factorization of finite state machines under observational equivalence -- Partial order logics for elementary net systems: State- and event-approaches -- Priority as extremal probability -- A synchronous calculus of relative frequency -- On the compositional checking of validity -- Real-time behaviour of asynchronous agents -- Effective solutions to domain equations an approach to effective denotational semantics.

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.