What Is The Longest Running Sci-Fi TV Series?
Answer: Dr. Who
As far as Sci-Fi series goes, Dr. Who is a veritable Methuselah. The show, focused on the adventures of The Doctor (a humanoid alien with extraordinary powers) and his inter-galactic adventures, first aired in 1963 in Britain.
It went on to run for 26 seasons, ending in 1989. In 1996 there was a TV film, and then, in 2005 the show was resurrected. The show is, as of this writing, still on the air and will soon be airing its 785th episode.
If you wanted to catch up on every episode, you’d need to take two months off work and log a solid shift of Dr. Who watching 7 days a week. Sadly, catching up on the entire series would be impossible as, thanks to poor archival practices at the BBC prior to 1978, 106 episodes were totally erased (so that the tapes could be reused) and 26 were partially erased. What is unique about the missing episodes, compared to other television series wiped by the BBC during that time period, is that devote fans recorded the episodes in audio format. Although the video footage may be missing for those hundred odd episodes, it is possible to listen to all of them.
Other notable long running Sci-Fi series include Star Trek (726 episodes across 5 television series and 1 animated series), Stargate (380 episodes across 3 television series and 1 animated series), Smallville (218 episodes), and The X-Files (202 episodes).