This book covers a broad range of important topics and recent developments in this field. First, the general language of quantum field theory is developed in a way appropriate for dealing with systems having a large number of degrees of freedom. This paves the way for a description of the basic processes in such systems, the emphasis being on phase transitions. Applications include various aspects of superfluidity and superconductivity, as well as a detailed description of the fractional quantum Hall liquid.This monograph addresses graduate students and researchers working in related disciplines looking for an approachable but thorough introduction to the field of condensed matter physics.