Chemical physics is presently a very active field, where theoretical computation and accurate experimentation have led to a host of exciting new results. Among these are the possibility of state-to-state reactive scattering, the insights in non-adiabatic chemistry, and, from the computational perspective, the use of explicitly correlated functions in quantum chemistry. Many of these present-day developments use ideas, derivations and results that were obtained in the very early days of quantum theory, in the 1920s and 1930s.Much of this material is hard to study for readers not familiar with German. This volume presents English translations of some of the most important papers. The choice of material is made with the relevance to present-day researchers in mind. Included are seminal papers by M Born and J R Oppenheimer, J von Neumann and E Wigner, E A Hylleraas, F London, F Hund, H A Kramers, R de L Kronig and F Hückel, among others.