This definitive book deals with the life and scientific work of arguably the greatest American-born theoretical physicist of the twentieth century. He was a great teacher, a born showman, bongo drummer, buffoon, and iconoclast; a scientific magician capable of transcendental leaps of the imagination. During his career he was drawn into research on the atomic bomb before working out his path-integral formulation of quantum mechanics and quantum electro-dynamics. Subsequently he developed the diagrammatic technique, as a result of which Feynman diagrams became ubiquitous in quantum field theory, elementary particle physics, and statistical mechanics. From 1950 he was based at the California Institute of Technology, where he worked on the superfluidity of liquid helium, the theory of polarons, the theory of weak interactions, the quantum theory of gravitation, partons, quark jets, and the limits of computation. Feynman had a unified view of physics and nature: he took the whole of nature as the arena of his science and imagination. Jagdish Mehra personally knew Feynman for thirty years. In 1980 Feynman suggested he might do what he had already done for Heisenberg, Pauli, and Dirac, that is write a definitive account of his life, science, and personality. Mehra instantly agreed and subsequently spent several weeks talking to him. After Feynman's death Mehra interviewed almost eighty people who had known him and aspects of his work. This book draws on this unique material and on Feynman's remarkable writings. It covers his childhood, his three marriages, his extraordinary range of interests. But most important, it deals with his scientific work in far greater detail than in any other biographical work on Feynman. What has emerged is a truly authoritative account of Feynman's life and achievements.