Physicists can't yet build time machines but they know that the laws of physics would allow one to exist. Scientists are investigating the manipulation of potential 'natural' time machines such as black holes, and other possibilities. All of these are discussed - not the detail of the mathematical equations, just the results - together with the implications of time travel, such as the 'granny paradox': a time traveller could go back in time, accidentally kill their granny, so the traveller's mother would never be born, so they would never be born, and so on. The Gribbins explain how such paradoxes are, in fact, prevented.