This book grew out of lectures that I gave in the framework of a graduate course in quantum theory of many-body systems at the Applied Physics Department of Chalmers University of Technology and G6teborg University (Geteborg, Sweden)in the years 1992-1995. Its purpose is to give a compact and self-contained account of basic ideas and techniques of the theory from the "condensed matter" point of view. The book is addressed to graduate students with knowledge of standard quantum mechanics and statistical physics. (Hopefully, physicists working in other fields may also find it useful.)