The Cybermen are humanoids who originally came from the planet Mondas who replaced their bodyparts with metal to survive. Their main weakness is gold. Thus, gold has frequently been used as a weapon against them.

They were the main enemy in Prison of Monsters and appeared in a flashback in Mission to the Sun. They would also have appeared in the Second Doctor's special Cybermen Invasion of Minecraftia.

The Cybermen were also the foes of the Doctor in an unknown adventure in Keralis, as seen in The White Guardian.

The foes returned a year later for the Valeyard, where they were the main enemy for the first 10 parts. They were defeated by the Doctor and the Edomites using golden bullets and explosives.