Quantum field theory is the basic mathematical framework that is used to describe elementary particles This textbook provides a complete & essential introduction to the subject Assuming only an undergraduate knowledge of quantum mechanics & special relativity this book is ideal for graduate students beginning the study of elementary particles The step-by-step presentation begins with basic concepts illustrated by simple examples & proceeds through historically important results to thorough treatments of modern topics such as the renormalization group spinor-helicity methods for quark & gluon scattering magnetic monopoles instantons supersymmetry & the unification of forces The book is written in a modular format with each chapter as self-contained as possible & with the necessary prerequisite material clearly identified It is based on a year-long course given by the author & contains extensive problems with password protected solutions available to lecturers at wwwcambridgeorg 9780521864497