The Doctor spends much of time saving the Earth (and other places) from various alien menaces.
His arch-enemies are the Daleks - metallic monsters which screech "Exterminate!" when they are about to shoot their victims.
The Doctor has also met up with two of his other most famous foes - the Cybermen and the Master.
The Cybermen have replaced most of their organs with machinery while the Master is an evil Time Lord.
Since the Doctor came back to our screens in 2005 he has clashed with lots of other unusual foes too.
Autons - shop dummies brought to life by the Nestene Consciousness.
Slitheen - huge, baby-faced aliens which cover themselves with human skin - eurghh!
The Ood - weird aliens who can't speak without translators but are able to communicate with each other by thought. Sadly that means other nasties can influence them with their minds too...
Weeping Angels - super scary aliens that turn to stone when you look at them making them look like statues. They attack when you're not looking at them, and send their victims to the past.