Life on other worlds, long a staple of sci-fi thrillers, is now the stuff of serious science. Aczel (Fermat's Last Theorem, 1996) brings a statistician's tools to bear on the evidence. Enrico Fermi once asked, ""Why, if there is intelligent life elsewhere, haven't they contacted us?"" One response to this paradox is Frank Drake's equation for the number of civilizations that might exist capable of communicating with other civilizations. For in a universe billions of years old, nearly infinite in extent, it's inconceivable that our world is the only one to be inhabited. And yet, flying-saucer cultists aside, there is no credible evidence of Earth's being visited by aliens. (One likely reason for this lies in the daunting distance between stars, which makes expeditions beyond a race's home system too expensive for even a very advanced civilization to undertake.) Most of the other evidence argues that there are planets around other stars, where the chemical reactions necessary for life (notably, the spontaneous formation of DNA) can take place. We also have evidence (in the form of Martian meteorites) that the spread of these chemicals from one world to another is moderately commonplace. Aczel stops to consider the central philosophical question raised by Einstein and the quantum theorists (i.e., whether the universe is random or deterministic); he clears up a few misconceptions about probability before applying probability theory to an assessment of the value of Drake's equation. His conclusion, that at least one planet besides Earth almost certainly bears life, will seem inevitable to anyone who has followed his arguments. Lively writing and the ability to work scientific history smoothly into the narrative make this a very useful addition to the growing body of work on a fascinating subject.