Richard Feynman devoted years to unravelling the fundamental problems of gravitation yet published comparatively little on the subject. These celebrated Caltech lectures brilliantly summarise his key ideas. Most were taped, typed up, revised and sold locally, where they exerted a huge influence on the next generation; now, at last, they are available to a wider readership in book form. Here Feynman explores some of his deep insights into cosmology, superstars, wormholes and gravitational waves, here that he offers the most accessible account of his pioneering approach to the foundations of general relativity. Presented with characteristic clarity, panache and intriguing philosophical digressions, these superb lectures give a vivid sense of a great scientist struggling to adapt accepted theory to new experimental evidence. They also provide a definitive analysis of gravitation at a time when Feynman and other leading physicists were revolutionising our understanding of the subject.