The question of whether or not we are alone in the universe has piqued the curiosity of humankind from ancient to modern times. 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 answers these questions in his engaging discussion of extraterrestrial intelligence. The author shows how planets fit into the hierarchy of the universe, and discusses 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 for searching and the results of current projects (including the Search for Extra Terrestrial Intelligence (SETI) project); the expansion of searches for extraterrestrials; the habitable zones in our universe; and what might happen if actual contact takes place. This lively account is written for a general readership with interest in this topic. Jean Heidmann is the author of Cosmic Odessey (CUP, 1989).