This study draws on the history and philosophy of science, cognitive psychology and the field of artificial intelligence to develop a theory of conceptual change capable of accounting for all major scientific revolutions. In addition to examining the Copernican and the Darwinian revolutions, Thagard turns his attention to the emergence of Newton's mechanics, Lavoisier's oxygen theory, Einstein's theory of relativity, quantum theory and the geological theory of plate tectonics. He discusses the psychological mechanisms by which new concepts and links between them are formed, and advances a computational theory of explanatory coherence to show how new theories can be judged to be superior to previous ones. To assess whether the field of psychology has undergone revolutions similar to those in physics, chemistry, biology and geology, he describes the nature and growth of behaviourism and cognitivism. Finally, he compares the transformations in scientific revolutions with the kinds of conceptual change that developmental psychologists have observed in children.