This is an introductory account of the physics of elementary particles and their interactions, with a minimum of formal apparatus and an ease of reading which, at present, is found in few other books in physics. It is designed for graduate students and for physicists not specializing in the field. The various phenomena are interpreted and correlated largely by means of elementary theoretical arguments needing little background beyond a first course in quantum mechanics. Numerous references to the original literature will allow the reader to probe more deeply into the topics discussed. Selected topics include scattering, photoproduction, K-mesons and hyperons, theoretical models, weak decay processes, and analysis of recent experiments on nonconservation of parity. Originally published in 1958. The Princeton Legacy Library uses the latest print-on-demand technology to again make available previously out-of-print books from the distinguished backlist of Princeton University Press. These editions preserve the original texts of these important books while presenting them in durable paperback and hardcover editions. The goal of the Princeton Legacy Library is to vastly increase access to the rich scholarly heritage found in the thousands of books published by Princeton University Press since its founding in 1905.