After introducing the empiricist point of view in philosophy of science, and the concepts and methods of the semantic approach to scientific theories, Professor van Fraassen discusses quantum theory in three stages. He first examines the question of whether and how empirical phenomena require a non-classical theory, and what sort of theory they require. He then discusses the mathematical foundations of quantum theory with special reference to developments in the modelling of interaction, composite systems, and measurement. Finally, the author broaches the main questions of interpretation. After offering a critique of earlier interpretations, he develops a new one - the modal interpretation - which attempts to stay close to the original Copenhagen ideas without implying a radical incompleteness in quantum theory. He gives special attention to the character of composite, many-body systems and especially to the peculiar character of assemblies of identical particles in quantum statistics.