This book presents an extended introduction to the theory of hadrons, the elementary particles that occur in the atomic nucleus. The main emphasis is on the theory of the complex angular momentum plane 'Regge theory'. In 1959 Tullio Regge demonstrated that it is useful to regard angular momentum as a complex variable when discussing solutions of the Schrödinger equation for non-relativistic potential scattering. This theory helps to classify the many different particles we know of, to explain the forces between them and to predict the results of high-energy scattering experiments. Regge...