Available for the first time in paperback, The Quantum Theory of Fields is a self-contained, comprehensive, and up-to-date introduction to quantum field theory from Nobel Laureate Steven Weinberg. The first volume introduces the foundations of quantum field theory, the second volume examines modern applications, and finally the third volume presents supersymmetry, an area of theoretical physics likely to be at the centre of progress in the physics of elementary particles and gravitation. The development is fresh and logical throughout, with each step carefully motivated by what has gone before. The presentation of modern mathematical methods is throughout interwoven with accounts of applications in both elementary particle and condensed matter physics. The three volumes contain much original material, and are peppered with examples and insights drawn from the author's experience as a leader of elementary particle research. Exercises are included at the end of each chapter.