This well-received work is now available in a new edition. It is an advanced text on quantum field theory--which is not only the accepted framework for describing all fundamental interactions except gravity, but also for understanding second-order phase transitions in statistical mechanics. The book approaches this subject in terms of path and functional integrals. A Euclidean metric has been adopted and the language of partition and correlation functions is used. Renormalization and the renormalization group are also discussed. Full mathematical details are provided. The text is intended for theoretical particle physicists and statistical physicists at the graduate level and above.