The parallel postulate of Euclidean geometry occupies a unique position in the history of mathematics. This is an account of the history of the development of Euclidean, non-Euclidean and relativistic ideas of the shape of the universe. The author reviews the failure of classical attempts to prove the postulate, before showing how the work of Gauss, Lobachevskii and Bolyai laid the foundations of modern differential geometry by constructing geometries in which the parallel postulate fails. The material, which has been revised and updated for this edition, includes a chapter on the Arabic contribution to mathematical history.