Most books about alien life are by astrophysicists, and spend their time arguing whether other planets and solar systems are habitable. This one is different. The authors point out that aqueous planets are common and where water exists life is likely. The test here is to find those features of life and evolution that are universal, and then to work out how they might have operated to produce life in other worlds. Along the way, the authors use examples of the aliens on offer via films, books and TV. Many have entirely reasonable biologies, others - cuddlies like ET, or dragons like the Alien - fail to measure up to the tests the authors set, but help narrow the search for the 'scientific alien'. The authors conclude that humans are not alone as intelligent entities, and that many others have appeared, and will appear, on other planets. And now, they argue, we can even surmise what they will look like and why.