List of the longest-running science fiction TV shows in US history. These sci-fi TV series have more episodes than any others in American television. Some of the shows ran and have since gone off the air or been canceled, while others remain ongoing and are still being produced. Typically, these shows were collected into "seasons" of television, each with a pre-set amount of episodes. Sometimes, these seasons were broken up, in some cases with years or even decades between production of the series, while still being considered the run of a single TV show. (Spin-offs and other shows that were reinvented and debuted as new series are not considered.) This list contains all the longest-running shows in the history of TV science fiction.
Typically, sci-fi has been a difficult genre for TV executives. Though many popular science-fiction shows have steadily built a die-hard following on TV, many of them are cancelled early, often due to the complications and cost of producing a weekly sci-fi adventure series that feels authentic and professional. Many of the sci-fi series now accepted as among the best of all time - from the original "Star Trek" to "Firefly" - have only lasted a few precious seasons.