This book has grown from lectures given in an advanced course in the theory of solids at the University of Illinois during the fall semester 1961-1962. It was assumed that the student possessed a knowledge of quantum mechanics equivalent to a years course in that topic; no prior knowledge of field theory was assumed. The course was designed to follow one in the one-electron or band theorv of solids. However, in the view of the wide applicability (to plasma physics, nuclear physics, low-temperature physics) of many of the physical ideas and techniques developed in the course, a prior knowledge of solid-siaie physics was not regarded as a sine qua non. For this reason, both Chapter 2 on phono ns and Chapter 3 on electrons contain a certain amount of material which is usually included in an introductory solid-state physics course.