The longest running science-fiction TV series is the BBC produced programme Dr. Who.
It has been aired since 1963 until today with a total of 783 episodes, 26 seasons (1963-1989), 6 series (2005-present) and one television film.
The Doctor has been played by 11 different actors being the transition of one doctor to the next part of the plot.
Currently it is played by Matt Smith that replaces David Tennant.
For more info: www.bbc.co.uk/doctorwho/dw