Asimov, who has written a virtual galaxy of excellent popular science books,… achieves something valuable [in Extraterrestrial Civilizations] by making a fresh, rigorously statistical analysis of the universe as we "know" it. In a sequence of short chapters he discusses possible habitable planetary systems that may be found in the cosmos; by well-argued processes of elimination he narrows his analysis down to a startling statement: "The number of planets in our galaxy on which a technological civilization is now in being" is roughly 530,000. For all the mathematical nature of his approach, Asimov's speculations are intriguing, although his closing guesswork on far-future cosmic exploration seems a papering over of our current state of ignorance.