The progress made in particle physics during the last two decades has led to the formulation of the so-called Standard Model of elementary particles and its quantitative experimental test. This book 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 is a comprehensive book incorporating all the topics for a unified treatment of particle physics. It provides good 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 nonrelativistic 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 (e.g. 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 flavors have been rewritten incorporating new material and new data. The heavy quark effective theory has been included.