This book is about spacetime, and how, in General Relativity, it performs its widely accepted role of explaining all the phenomena of gravity. Space and time have long been contentious presences in theories of mechanics because of their metaphysical peculiarities. Philosophical worries continue to fog a clear understanding of just how spacetime performs this role. In early sections of his revolutionary paper of 1916, Einstein claimed that his use of a new style of formulation removes "the last remnant of physical objectivity from space and time". His satisfaction in banishing these metaphysical anxieties was evident. However in 1917 it was shown his claim was ungrounded. He readily accepted the criticism: it left intact all the maths and physics of the theory. These amply justify the widespread, orthodox view of spacetime's fundamental role in the theory. Einstein banished no demon but rather released a genie - spacetime. However, his hostility continued, resulting in attempts to rewrite the role of spacetime. These had some later influence but never caught on. Einstein set supreme importance on the role of imagination in scientific theory and valued it as his most significant gift. In science, perhaps in culture generally, relativity is unsurpassed in its imaginative conceptual daring. Yet he and others paused at its metaphysical novelty and radicalism: hostility to spacetime persists. This book presents an unqualified philosophical defence of the conceptual coherence and uniqueness of spatiotemporality. It offers an alternative to the relationist/substantivalist dilemma; it explains Minkowski's derivation of spacetime without the light postulate; it defends, clarifies and stresses the deep role of spacetime in the standard novelties of both relativity theories. A study of spacetime in relativity shows how an insubstantial entity can nevertheless be concrete. That solves the metaphysical worry over its intelligibility and reveals it as fundamental to ontology.