Infinity is surely the strangest idea that humans have ever thought. Where did it come from and what is it telling us about our Universe? Can there actually be infinities? Or is infinity just a label for something that is never reached, no matter how long you go on counting? Are infinities like numbers, with some bigger than others, and one infinity at the top, bigger than all the rest? Can you do an infinite number of things in a finite amount of time? Is the universe infinite? Is it infinitely old and will it continue to exist forever? Is matter infinitely divisible into ever-smaller pieces? But infinity is also the place where things happen that don't. All manner of strange paradoxes and fantasies characterise an infinite universe. If our Universe is infinite then an infinite number of exact copies of you are at this very moment reading an identical sentence on an identical planet somewhere else in the Universe. So what is it like to live in a Universe where nothing is original, where you can live forever, where anything that can be done, is done, over and over again? These are some of the deep questions that the idea of the infinite pushes us to ask. Throughout history, the infinite has been a dangerous idea. Many have lost their lives, their careers, or their freedom for talking about it. The Infinite Book will take you on a tour of these dangerous questions and the strange answers that scientists, mathematicians, philosophers, and theologians have come up with to deal with its threats to our sanity.