The recent discoveries of extrasolar planets and possible microfossils in Martian meteorite ALH 84001 are only the latest developments in a debate that spans millennia, and that has been especially heated in the twentieth century. From the furore over Percival Lowell’s claim of canals on Mars at the beginning of the century to the biological experiments of the Viking spacecraft, the controversial ‘Mars rock’, and the sophisticated Search for Extraterrestrial Intelligence (SETI) at its end, otherworldly life has often titillated and occasionally consumed science and the public. So too have crucially related areas such as the search for planetary systems, the quest for an explanation of UFOs, and inquiries into the origin of life. The theme has been elaborated in science fiction from H. G. Wells to Arthur C. Clarke, and resulted in some of the most popular films of all time, including ET, Aliens, Independence Day, and Contact. Life on Other Worlds details in a readable and non-technical manner the history of the twentieth century extraterrestrial life debate, one of the pervasive themes of our century.