Acknowledgements

AbstractThis monograph is a revison of author's Ph.D. thesis [Bon96]. First of all, I wish to express my gratitude to my thesis supervisors Jaco de Bakker and Joost Kok. I am particularly grateful to them for their advices, enouragements, and suggestions which still have a great influence on my work. I am thankful to Michael Mislove for critically reading my thesis and for his helpful comments.Many thanks are due to my colleagues Stefan Blom, Mirna Bognar, Frank de Boer, Henk Goeman, Jan Willem Klop, Vincent van Oostrom, Daniele Turi, Yde Venema, and Roel de Vrijer, not only for the stimulating discussions we had but also for their friendliness. I am particularly grateful to Franck van Breugel, Joost Kok, Bart Jacobs, Erik de Vink, Marta Kwiatowska, and Jan Rutten, for a very fruitful and enjoyable collaboration.I thank Samson Abramsky, Ralph Back, Bob Flagg, Paul Johnson, Wim Hesselink, Peter Knijnenburg, Paul-André Mellies, Prakash Panangaden, Mike Smyth, Philipp Sünderhauf, and Fer-Jan de Vries for the many inspiring discussions we had in various places around the world.A word of thanks goes also to the people who invited me to discuss parts of the results presented here: Maurice Nivat and Paul Gastin of the Université de Paris, Marta Kwiatowska of the University of Birmingham, and Willem-Paul de Roever and Kai Engelhart of the Christian-Albrechts Universitaet zu Kiel. During my visits to them I benefited from many fruitful discussions and I had a very enjoyable time.Finally, I thank my parents and my wife for their persisting encouragements and support.Marcello BonsangueDepartment of Computer ScienceLeiden UniversityP.O. Box 95122300 RA Leiden, The NetherlandsPhD ThesisVrije UniversiteitAmsterdamCommuncating Editor: Michael W. Mislov