This is a welcome reissue, with a new Preface, of John S. Rigden's stellar biography of I. I. Rabi, one of the most influential physicists of the twentieth century. Rabi's discovery of the magnetic resonance method won him the Nobel Prize in 1944 and stimulated research leading to, among other things, refinements in quantum electrodynamics, refined molecular beam methods, radio astronomy with the hydrogen 21-cm line, atomic clocks, and solid state masers.