Following on from the successful first (1984) and revised (1993) editions, this extended and revised text is designed as a short and simple introduction to quantum field theory for final year physics students and for postgraduate students beginning research in theoretical and experimental particle physics.<br><br> The three main objectives of the book are to:<br><br> Explain the basic physics and formalism of quantum field theory<br><br> To make the reader proficient in theory calculations using Feynman diagrams<br><br> To introduce the reader to gauge theories, which play a central role in elementary particle physics.<br><br> Thus, the first ten chapters deal with QED in the canonical formalism, and are little changed from the first edition. A brief introduction to gauge theories (Chapter 11) is then followed by two sections, which may be read independently of each other. They cover QCD and related topics (Chapters 12-15) and the unified electroweak theory (Chapters 16 - 19) respectiv