Theory of Superconductivity is considered one of the best treatments of the field. This monograph, by Nobel Prize-winning physicist J. Robert Schrieffer, has been reprinted because of its enduring value as an introduction to the theory of superconductivity. Based on a series of lectures presented by the author at the University of Pennsylvania, the fundamentals of the theory of superconductivity are stressed as a means of providing the reader with a framework for the literature in which detailed applications of the microscopic theory are made to specific problems. It also serves as a foundation for the more recent developments in this active field.