Grand Unified Theories introduces the application of gauge field theories to a unified description of the strong, electromagnetic, weak, and gravitational interactions. The phenomenological aspects of the work are emphasized and explicit calculations presented. Many of the aspects of current research, including technicolor models, supersymmetry and supergravity, and the cosmological implications of these theories, are discussed in this book.This book is suitable for graduate students with a background in quantum mechanics, and experimental and theoretical particle physicists who want to understand the grand unified theories. It is part of the Frontiers in Physics series, edited by David Pines.