What is time? The 5th-century philosopher St Augustine famously said that he knew what time was, so long as no one asked him. Introducing Time tackles this question and dares to go where Augustine would not. It traces the history of time from Augustine's suggestion that there is no time, to the flowing time of Newton, the conventional time of Poincare, the static time of Einstein, and then back, full circle to the idea that there is no time in quantum gravity. Along the way, many puzzling questions are raised. For instance, is time a fourth dimension similar to space or does it 'flow' in some sense? And if it flows, does it make sense to say how fast? Does the future exist? Is time travel possible? Why does time seem to pass in only one direction? These questions and others are among the deepest and most subtle that one can ask, but this book presents them - many for the first time - in an easily accessible, lucid and engaging manner.