The universe was supposed to be all there was - the whole of existence. For centuries the size of the universe and whether or not it is finite, fascinated scientists and philosophers. But however big the universe may be, there being only one gave astronomers and cosmologists a headache. Existence as we know it is contingent on so many constants in nature which, if altered only slightly, would make life impossible. This seems so unlikely to happen by chance. What if, instead, there is in fact a vast range of different universes, with many variants of the universal contents, making ours just one of many that happens to be viable for life? In this illuminating exploration, Brian Clegg delves into the endless possibilities of the multiverse. Investigating questions of expanding, parallel and communicating universes, this accessible guide takes the reader into one of the weirdest and most exciting areas of modern physics and cosmology.