During the past twenty five years our knowledge of the structure and properties of atoms and molecules has been increased considerably by the progress made in both the theoretical and experimental methods of approach to the problems involved. The introduction of wave mechanics has permitted the development of theories of atomic and molecular structure which have placed on a theoretical basis such fundamental concepts as the spatial configuration of atomic orbitali, the nature and directional characteristics of bonds between atoms in molecules and the mechanism of chemical reactions. However, calculations of bond strengths and distances and of activation energies are possible only with the simplest systems and perhaps the greatest contribution of wave mechanics to chemistry up to the present has l>een the introduction of ideas which, by general application to the experimental data for complex molecules, has increased considerably our understanding of the structure of such molecules and the reactions which they undergo. Therefore it is necessary that in any treatise on molecular structure, the theoretical and the experimental aspects of the problem should be considered side by side. This is what the authors have attempted in this book. In the early chapters the wave mechanics of simple atoms and molecules are discussed ; all the mathematical derivations are collected in the last three chapters of the book apart from the Heitler-I-ondon treatment of the homopolar Ixmd, which the authors consider should be familiar to all who desire to understand the present position regarding the theory of the chemical bond and which is presented in a form such that the necessary approximations and simplifications may be appreciated readily. The later chapters are confined to a survey of the experimental data relevant to a study of molecular structure and to the theoretical conclusions which may be drawn therefrom.