At the present time a number of particles is known which corre­ spond to various quantum fields interacting with each other. However, of the many types of physical interactions existing in nature the only one, apart from gravitation, that has been studied in sufficient detail is the electromagnetic interaction. The theory of the latter interaction is the subject of quantum electrodynamics, to which the systematic exposition of this book is devoted.Since the electromagnetic interaction is the fundamental one in the case of electrons and photons, quantum electrodynamics enables us to explain and to predict a wide range of phenomena related to the behavior of these particles. In preparing the second edition we have subjected the book to extensive revision. The prineipal aim and the contents of the book have not been altered; the book is devoted to the systematic presen­ tation of electromagnetic processes only. Only some general theorems and methods go beyond the framework of electrodynamics proper. In the second edition the space devoted to these has been increased(reflection properties, Green’s functions, functional methods, etc.).In the presentation of the principles of quantum electrodynamics the theory of renormalizations has been subjected to the most exten­ sive revision. Without claiming complete mathematical rigor we have attempted to present the concept of renormalization from a single simple physical point of view, avoiding purely prescription-like methods of eliminating divergences, and making maximum use of the general properties of quantum mechanical systems.