Interest in exoplanets--the worlds of other stars--is not new. From the late 17th century until the end of the 19th, almost all educated people believed that the stars are suns surrounded by inhabited planets--a belief that was expressed not in science fiction, but in serious speculation, both scientific and religious, as well as in poetry. Only during the first half of the 20th century was it thought that life-bearing exoplanets are rare. This is not a science book--rather, it belongs to the category known as History of Ideas. First published by Atheneum in 1974, it tells the story of the rise, fall, and eventual renewal of widespread conviction that we are not alone in the universe. In this 2012 updated edition the chapters dealing with modern speculation have been revised to reflect the progress science has made during the past 40 years, including the actual detection of planets orbiting other stars. However, it is not intended to be more than a brief introduction to today's views; its focus is on little-known facts about those of the past. Why should we care what our forebears believed? Now, the question of ET life is a matter for investigation by science. Yet it's significant that most educated people of past centuries were convinced that other inhabited worlds exist, without any scientific evidence whatsoever. This historical fact reveals that human beings have an instinctive sense of kinship with the wider universe and a desire to see the realms that lie beyond this one small planet--and perhaps, eventually, to go there. Our ancestors conceived of such voyages only in a spiritual sense, as occurring after death. But we who have taken our first small steps into space are aware that our descendants may set foot on the worlds of other suns. Just as in the 17th century people were initially upset by the new knowledge that the stars are suns scattered in space rather than lights fixed to a nearby sphere, the growing awareness that Earth is not safely isolated from whatever lies beyond makes many of our contemporaries uneasy. Thus today's predominant feelings about spaceships are ambivalent. Nevertheless, if an impulse toward belief that we are not alone in the universe is indeed an innate characteristic of human beings, as the past spread of belief in inhabited exoplanets suggests, we can be sure that those who follow us will not turn back from becoming spacefarers.