Extraterrestrial life is a common theme in science fiction, but is it a serious prospect in the real world? Astrobiology is the emerging field of science that seeks to answer this question. The possibility of life elsewhere in the cosmos is one of the most profound subjects that human beings can ponder. Astrophysicist Andrew May gives an expert overview of our current state of knowledge, looking at how life started on Earth, the tell-tale 'signatures' it produces, and how such signatures might be detected elsewhere in the Solar System or on the many 'exoplanets' now being discovered by the Kepler and TESS missions. Along the way the book addresses key questions such as the riddle of Fermi's paradox ('Where is everybody?') and the crucial role of DNA and water – they're essential to 'life as we know it', but is the same true of alien life? And the really big question: when we eventually find extraterrestrials, will they be friendly or hostile?