This biennial Review summarizes much of Particle Physics. Using data from previous editions, plus 1726 new measurements from 512 papers, we list, evaluate, and average measured properties of gauge bosons, leptons, quarks, mesons, and baryons. We also summarize searches for hypothetical particles such as Higgs bosons, heavy neutrinos, and supersymmetric particles. All the particle properties and search limits are listed in Summary Tables. We also give numerous tables, figures, formulae, and reviews of topics such as the Standard Model, particle detectors, probability, and statistics. Among the 119 reviews are many that are new or heavily revised including those on neutrino mixing, CP violation in K, D, and B mesons, Vcb, the new exotic Θ(1540) particle, extra-dimensions, grand unified theories, cosmic background radiation, dark matter, cosmological parameters, and big bang cosmology. A booklet is available containing the Summary Tables and abbreviated versions ofsome of the other sections of this full Review. All tables, listings, and reviews (and errata) are also available on the Particle Data Group website: http//pdg.lbl.gov. ©2004 Regents of the University of California.