Only about seven years after quantum mechanics obtained its final form W. Pauli, himself one of the most active and most critical pioneers of the new theory, wrote this classic. He revised it in 1958 for the Enclyclopedia of Physics and it was this version from which this translation was made. More than any other book on quantum mechanics this text works out its foundations, which explains the continuing interest of the physics community in this book. For their historical value the translators included also the sections on quantum electrodynamics from the 1933 edition.