If extraterrestrial intelligence exists, then positive detection of it would be the greatest scientific discovery of all time. By what criteria should we judge if we are alone in the cosmos, and how should we set about detecting extraterrestrials? Jean Heidmann and Strom Dunlop answers these questions in this engaging discussion of extraterrestrial intelligence. The authors shows how planets fit into the hierarchy of the Universe, and discuss prebiotic stages of life, and the emergence of primitive biological life in the solar system. From this base the entire subject of extraterrestrials is explained: techniques and the results of projects, the expansion of searches for extraterrestrials, the habitable zones in our universe, and what might happen if actual contact takes place. This is a lively account, written at a level suitable for general readership.