This is the first in-depth study in English of the international debate that developed between 1750 and 1900 concerning the question of the existence of extraterrestrial intelligent life. Taking a history of ideas approach, the author describes the controversies that-arose over this question and reveals the great extent to which this issue influenced astronomical, philosophical, and religious thought. Professor Crowe shows that the majority of the leading astronomers of the last two centuries participated in this debate and he analyzes how their views interacted with new developments such as Newtonian mechanics, stellar astronomy, Darwinian theory, and astrophysics. This fascinating and critical history shows that the longstanding and widespread belief in extraterrestrial life has for centuries acted to alter major areas of our intellectual life.