This monograph introduces modern developments on the bound state problem in Schr?dinger potential theory and its applications in particle physics. The Schr?dinger equation provides a framework for dealing with energy levels of N-body systems. It was a cornerstone of the quantum revolution in physics of the twenties but re-emerged in the eighties as a powerful tool in the study of spectra and decay properties of mesons and baryons. This book begins with a detailed study of two-body problems, including discussion of general properties, level ordering problems, energy level spacing and decay properties. Following chapters treat relativistic generalisations, and the inverse problem. Finally, 3-body problems and N-body problems are dealt with. Applications in particle and atomic physics are considered, including quarkonium spectroscopy. The emphasis throughout is on showing how the theory can be tested by experiment. Many references are provided.