Quantum electronics as an independent field of physics came into prominence in the middle of the fifties when the first quantum oscillators and amplifiers were made. This field at once attracted the attention of a large number of research workers, and rapid progress took place. The present book attempts to give a resume of this material and to a certain extent, to generalize it. The material has been arranged as such that the reader need not continually refer elsewhere. In Volume 1, the authors gave the basic theory of quantum electronics. In this part, they have tried to show how the concepts and equations used in quantum electronics follow from the basic principles of theoretical physics. When doing this, they made frequent use of very simple models so as not to complicate the treatment. Research on the subject has of course continued since the publication of this book. The material covered in this edition, however, will form a solid foundation for anyone with an academic interest in the field of quantum electronics.