This monograph presents a coherent and elementary introduction to Gauge theories of the fundamental interactions and their applications to high-energy physics. It deals with the logic and structure of local Gauge symmetries and Gauge theories, from quantum electrodynamics through unified theories of the interactions among leptons and quarks. Many explicit calculations provide the reader with practice in computing the consequences of these theories and offer a perspective on key experimental investigations. First published in 1983, this text is ideal for a one-semester course on Gauge theories and particle physics. Specialists in particle physics and others who wish to understand the basic ideas of Gauge theories will find it useful as a reference and for self-study.