Julian Schwinger‎,‎ who shared the 1965 Nobel Prize for physics with Richard Feynman and Sin‎-Itiro Tomonaga for his pioneering work on quantum electrodynamics‎,‎ had a considerable influence on the conceptual development of modern quantum field theory‎.‎ In addition to being an extremely productive researcher he was also a brilliant teacher‎,‎ and this book demonstrates his outstanding ability to expose a difficult subject in a clear and concise style‎.‎ In marked contrast to many textbooks on quantum physics‎,‎ the whole of quantum kinematics and the underlying quantum action principle‎,‎ which now bears his name‎,‎ are derived through a systematic analysis of experimental phenomena‎,‎ thus avoiding the ad hoc postulates of the axiomatic approach‎.‎ Many applications‎,‎ all worked out in detail‎,‎ follow and culminate in an introduction to quantum electrodynamics‎.‎ A unique legacy‎,‎ these lecture notes of Schwingers course held at the University of California at Los Angeles were carefully edited by his former collaborator Berthold‎-Georg Englert and constitute both a self‎-contained textbook on quantum mechanics and an indispensable source of reference on this fundamental subject by one of the foremost thinkers of twentieth century physics‎.‎