The progress made in particle physics during the last two decades of the 20th century has led to the formulation of the so-called Standard Model of elementary particles and its quantitative experimental test. This work presents that progress, and also includes chapters which provide background on modern particle physics. Particle physics forms an essential part of the physics curriculum. This book seeks to incorporate all the topics for a unified treatment of the subject. It provides reference material for researchers in both theoretical and experimental particle physics. It is designed as a semester course for senior undergraduates and for graduate students. Formal quantum field theory is not used. A knowledge of non-relativistic quantum mechanics is required for some parts of the book, but for the remaining parts familiarity with the Dirac equation and Feynman rules is essential. However, some of these topics are included in an appendix. In this second edition, many chapters (for example, on electroweak unification) have been revised to bring them up to date. In particular, the chapters on neutrino physics, particle mixing and CP violation, and weak decays of heavy flavours have been rewritten incorporating new material and new data since the first edition. The heavy quark effective theory has been included.