ACP, the Algebra of Communicating Processes, is an algebraic approach to the study of concurrent processes, initiated by Jan Bergstra and Jan Will em Klop in the early eighties. These proceedings comprise the contributions to ACP94, the first workshop devoted to ACP. The work shop was held at Utrecht University, 16-17 May 1994. These proceedings are meant to provide an overview of current research in the area of ACP. They contain fifteen contributions. The first one is a classical paper on ACP by J.A. Bergstra and J.W. Klop: The Algebra of Recursively Defined Processes and the Algebra of Regular Processes, Report IW 235/83, Mathematical Centre, Amsterdam, 1983. It serves as an introduction to the remainder of the proceedings and, indeed, as a general introduction to ACP. An extended abstract of this paper is published under the same title in the ICALP' 84 proceedings. Of the re maining contributions, three were submitted by the invited speakers and the others were selected by the programme committee. As for the presentations, Jos Baeten, Rob van Glabbeek, Jan Friso Groote, and Frits Vaandrager were each invited to deliver a lecture. A paper relating to Frits Vaandrager's lecture has already been submitted for publication elsewhere and is not, therefore, included in these pro ceedings. Gabriel Ciobanu, one of our guests, gave an impression of his work in an extra lecture. Furthermore, ten presentations were given on the basis of selected papers."

Labelled transition systems are mathematical models for dynamic behaviour, or processes, and thus form a research field of common interest to logicians and theoretical computer scientists. In computer science, this notion is a fundamental one in the formal analysis of programming languages, in particular in process theory. In modal logic, transition systems are the central object of study under the name of Kripke models. This volume collects a number of research papers on modal logic and process theory. Its unifying theme is the notion of a bisimulation. Bisimulations are relations over transition systems, and provide a key tool in identifying the processes represented by these structures. The volume offers an up-to-date overview of perspectives on labelled transition systems and bisimulations.