Quantum theory is used to describe the physics of elementary particles, and has applications in many areas of particle and condensed matter physics. This textbook is a complete and essential introduction to the theory of elementary particles. It is written in a modular format, and each chapter is self-contained. The prerequisite material needed for each chapter is listed, giving the book great flexibility. It covers all the key theories necessary to understand the standard model and contains treatments of modern topics, including the renormalization group, spinor-helicity, methods for quark and gluon scattering, magnetic monopoles, instantons, supersymmetry, and unification forces. Assuming an undergraduate knowledge of quantum mechanics and special relativity, this book is ideal for graduate students studying quantum field theory and elementary particle theory.