The molecular structure hypothesis — that a molecule is a collection of atoms linked by a network of bonds — provides the principal means of ordering and classifying observations in chemistry. However this hypothesis is not related directly to the physics which governs the motions of atomic nuclei and electrons. It is the purpose of this important new book to show that a theory can be developed to establish the molecular structure hypothesis, demonstrating that the atoms in a molecule are real, with properties predicted and defined by the laws of quantum mechanics, and that the structure their presence imparts to a molecule is indeed a consequence of the underlying physics. As a result, the classification based upon the concept of atoms in molecules is freed from its empirical constraints and the full predictive power of quantum mechanics can be incorporated into the resulting theory — a theory of atoms in molecules. Eminently accessible and readable, the book will interest all scientists involved with experiment and observation at the atomic level, in addition to theoreticians.