The logic of theory change, or the theory of belief revision, is a growing area of research in artificial intelligence. The papers in this volume, mostly arising from a workshop held in Konstanz in October 1989, chart out the current state of research in this area and indicate the directions of future research. Connections to other areas of AI research are emphasized throughout the book. All the contributions are original research papers and most of them are self-contained, so that the collection will be of interest not only to researchers but also to all those who seek an introduction to the state of the art in this area. The fourteen contributions are organized into five sections: - Foundations - Models of theory change - The Ramsey test for conditionals - Theory change and nonmonotonic reasoning - Reformulations and extensions.