The Foundation series is a science fiction series by Isaac Asimov. For nearly thirty years, the series was a trilogy: Foundation, Foundation and Empire, Second Foundation. Asimov began adding to the series in 1981, with two sequels: Foundation's Edge, Foundation and Earth, and two prequels: Prelude to Foundation, Forward the Foundation. The additions made reference to events in Asimov's Robot and Empire series, indicating that they were also set in the same fictional universe. More: en.wikipedia.org.