Astrobiology, the study of life and its existence in the universe, is now one of the hottest areas of both popular science and serious academic research, fusing biology, chemistry, astrophysics, and geology. In this masterful introduction, Lewis Dartnell explores its latest findings, and delves into some of the most fascinating questions in science. What actually is 'life'? Could it exist on other planets? Could alien cells be based on silicon rather than carbon, or need ammonia instead of water? Introducing some of the most extreme lifeforms on Earth - those thriving in boiling acid or huddled around deep-sea volcanoes - Dartnell takes us on a tour of the universe to reveal how deeply linked we are to our cosmic environment, and shows why the Earth is so uniquely suited for the development of life.