The most comprehensive work of its kind, "Great Themes of Science Fiction" looks at the most important elements of the genre and shows how each has evolved over time. Among the topics examined are aliens and alien worlds, supermen and adapted men, immortality and paranormal powers, robots and computers, space civilizations and colonies, wars on earth and in space, worldwide disasters, time travel, and parallel worlds. Selected examples of each theme are discussed together with changes that have become apparent with shifts in popular attitudes and the increasing sophistication of readers. The impact and significance of each type of science fiction scenario is considered, and the challenges it poses to human ingenuity, understanding, and responsibility are discussed.