This book is a comprehensive introduction to particle physics, bridging the gap between traditional textbooks on the subject and popular accounts that assume little background knowledge. This fourth edition is fully revised, including the most recent ideas and discoveries, and the latest avenues of research. The development of the subject is traced from the foundations of quantum mechanics and relativity, through the formulation of quantum field theories, to the standard model. Research now continues with the first signs of physics beyond the standard model and with the formulation of modern string theory which aims to include a quantum theory of gravity for the first time. This book is intended for anyone with a background in physical sciences who wishes to learn about particle physics. It is also valuable to students of physics wishing to gain an introductory overview of the subject.