I would like to use this preface to thank some persons and institutions which have been important during the various stages of writing this book. First of all, I am grateful to Kluwer Academic Publishers for giving me the opportunity to write this book. I started writing the book in 1998 while I was working at the Departament d'Economia i d'Historia Economica at Universidad Autonoma de Barcelona, and continued the writing job from september 1998 to september 2000 at the Departamento de Economfa at Universidad Carlos III de Madrid. The book has been completed while I was visiting the Department of Quantitative Economics at the University of Maastricht from october 2000 to august 2001. I wish to thank these three departments for their hospitality. The book has improved substantially by comments and critique from the following persons who have read parts of the manuscript: Geir Asheim, Eric van Damme, Janos Flesch, Mari-Angeles de Frutos, Diego Moreno, Hans Peters, Antonio Romero and Dries Vermeulen. I should also mention my discussions with Peter Wakker about the decision-theoretic foundations of noncooperative game theory, which have had an important impact on various parts of the book. Finally, I wish to express my warmest gratitude to my parents, my brother and my sister, and, last but not least, to Cati, to whom I dedicate this book.