This book was written in the years 1989 and 1990 at the Centre for Mathematics and Computer Science in Amsterdam. Much of the material comes from a Dutch book (reference Baeten [1986]), that was used in university courses at the University of Amsterdam and the University of Utrecht.

For the first time, this book brings together a lot of work in the area of algebraic process theory, as this has been developed since 1982 by a group of people in Amsterdam and Utrecht under the guidance of Jan Bergstra. Most of the research that found its way into this book was funded by European programmes, notably METEOR (ESPRIT I), SPECS (RACE), CONCUR (ESPRIT II BRA).

Many people contributed in one way or another to this book. We want to mention (in alphabetical order) Gert-Jan Akkerman, Jan Bergstra, Wiet Bouma, Jeroen Bruijning, Nicolien Drost, Rob van Glabbeek, Jan Friso Groote, Jan Willem Klop, Karst Koymans, Evangelos Kranakis, Sjouke Mauw, Kees Middelburg, Hans Mulder, Eric Nieuwland, Ernst-Rudiger Olderog, Alban Ponse, Scott Smolka, Frits Vaandrager, Gert Veltink, Jos Vrancken, Han Zuidweg. Thanks are due to the head of the Department of Software Technology of the Centre for Mathematics and Computer Science, Jaco de Bakker, who created an excellent working atmosphere.

The first author wants to dedicate this book to his wife Jeanne, who was very important in its writing, but cannot see its completion.