This is a new edition of Gärdenfors' classic text, presented to the community with a Foreword by David Makinson and an appendix containing a paper "Relations between the logic of theory change and nonmonotonic logic" by the author and David Makinson. The following describes the first edition: Knowledge in Flux presents a theory of rational changes of belief, focusing on revisions that occur when the agent receives new information that is inconsistent with the present epistemic state. It brings together, systematises and enlarges upon an already influential body of work by the author and his colleagues on the dynamics of theories and epistemic states. The problem of knowledge representation is one of the most important current research problems in philosophy, artificial intelligence, and cognitive science in general. While most of the research has been devoted to analysing the structure of epistemic states, this book is unique in describing the dynamics of knowledge and belief, and in presenting models of knowledge that focus on expansions, revisions, and contractions of epistemic states. "Knowledge in Flux is sure to be widely recognised as the most important systematic contribution to these issues yet made." David J. Israel, Senior Computer Scientist, SRI International Peter Gärdenfors is a Professor in Cognitive Science at Lund University.