Taking into account recent experimental progress, this revised and updated edition of a classic textbook provides an accessible and practical introduction to the three gauge theories included in the Standard Model of particle physics: quantum electrodynamics (QED), quantum chromodynamics (QCD), and the Glashow-Weinberg-Salam (GWS) electroweak theory. For each theory, the authors discuss the main conceptual points, detail many practical calculations of physical quantities from first principles, and compare these quantitative predictions with experimental results. The text emphasizes the development of both calculation skills and physical insight.