This volume collects outstanding contributions to the theory of games, the theory of game-theoretical rationality, and their applications. 27 articles present the new situation and the recent advances in game theory after the award of the Nobel Prize in economics and especially in game theory to John F. Nash, John C. Harsanyi, and Reinhard Selten. Two of them, Harsanyi and Selten, have contributed leading articles to this volume. In utility and game theory, the question of which rationality governs their methods and the behavior of the agents as well has emerged as one of the most exciting new conceptual foundations of all social sciences. The main aim of this book is to find an answer to this problem. Do we have to give up our belief in the traditional form of deductive and linear rationality in the social sciences in favor of probabilistic and stochastic methods? Which kind of rationality do we, and should we, use when we attempt to practically solve societal problems and conflicts? Quite a few articles in this book address these questions. The consequences of a new, multi-faceted rationality, which is going to shake the traditional foundation of game theory, decision theory, and utility theory, and, finally, the social sciences in their entirety, are discussed in depth in seven chapters and a preface: ‘Rationality and the Foundations of the Social Sciences,’ ‘Cooperation and Rationality,’ ‘Rationality and Economics,’ ‘Bayesian Theory and Rationality,’ ‘Evolution and Evolutionary Game Theory,’ ‘Ethics and Game Theory,’ and ‘Applications of Game Theory’. The contributors include economists, utility and decision theorists, psychologists, sociologists, physicists, philosophers of sciences and probability theorists. They attempt to make their contributions accessible to a wide audience. The book will interest researchers, teachers and advanced students in the above-mentioned disciplines; it can be used for a one-semester course on the graduate level. The volume also includes a review section focusing on recent publications on Logical Empiricism and its influence. An autobiographical report on the Vienna Circle by Arne Naess follows the main part of the Yearbook. An overview of the activities of the Institute Vienna Circle 1997/98 concludes the volume.