The demands upon experimental researchers in chemistry and allied disciplines for theoretical understanding of their experiments has grown rapidly in recent decades. In many cases, the theories of atomic and molecular structure and spectroscopy for example, the demand is for quantum mechanics. Through no fault of their own, many such workers lack a background in this widely-feared subject and are looking for ways in which they can develop their knowledge in that area in order to be able to interpret their own experimental results and plan new experiments with greater insight. Similarly, leaders of research groups, particularly but not solely in academia, are looking for books which they can recommend to their students, post-doctoral fellows and colleagues with a view to improving their understanding of their work by individual study or in small research study-groups. This book attempts to remedy this situation by providing an account of the quantum theory and some of its applications to chemistry that is addressed to experimentalists and tries to answer their problems. The book describes, in as full and understandable a way as possible, the fundamentals of the quantum theory laying particular emphasis upon the way in which experimental measurements and the theoretical interpretations of them are linked, to assist such research workers, who could be third-year project students, postgraduate students, faculty members or researchers in industrial/commercial/governmental laboratories, to come to terms with these concepts.