The Slitheen are a ruthless criminal sect from the planet Raxacoricofallapatorius whose main motivation is profit. They are instinctive hunters, being trained to hunt and kill from a young age. The members of the family are convicted criminals on their planet, being subject to the death penalty.

The Slitheen disguise themselves by fitting into the skins of their victims, using compression fields created by a collar worn around their necks to squeeze their huge size into a slightly smaller space. Initially the compression ratio was limited, so the disguises tended to be that of obese people.

The exchange of gases that compression entails also builds up within the acquired skin, causing a condition similar to flatulence in humans (the expelled gas smells like bad breath, which the Doctor noted was a form of calcium decay, although in reality the bacteria that cause tooth decay are different from those that cause bad breath).

Raxacoricofallapatorians are vulnerable to acetic acid, which reacts explosively—and fatally—with their bodies