This is a textbook of advanced quantum theory for graduate students and research workers which gives a connected mathematical derivation of the important results, concentrating on the central ideas without burdening the exposition with elaborate detail or unnecessary rigour, and explains, in the simplest possible terms, the symbols and concepts which frequently confront the active research worker in solid state, nuclear and high-energy physics, and in theoretical chemistry. Professor Ziman brings to his task the sympathetic guidance of a lecturer who has not forgotten the difficulties that he himself had to surmount in mastering his subject.