Quantum field theory forms the present theoretical framework for our understanding of the fundamental interactions of particle physics. This updated and expanded volume examines gauge theories and their symmetries with an emphasis on their physical and technical aspects. A new introductory chapter gives a systematic overview to classical field theories and a short discussion of their canonical quantization and the discrete symmetries C, P and T. Coverage provides a brief exposition of perturbation theory, the renormalization program, and the use of the renormalization group equation. It then explores topics of current research interest including chiral symmetry and its breaking, anomalies, and low energy effective Lagrangians and some basics of supersymmetry. A chapter on the basics of the electroweak theory is also new to this edition, as well as an appendix that contains a complete set of Feynman rules for the Standard Model. This volume will be ideal for graduate students and researchers in theoretical physics, condensed matter physics, and applied mathematics.