The Slitheen are a ruthless extraterrestrial crime family whose main motivation is profit, but they have an almost ritualised love of hunting, being trained to hunt and kill from a young age. The members of the family are convicted criminals on their home planet of Raxacoricofallapatorius, being subject to the death penalty if they return.
Raxacoricofallapatorians are massive bipedal creatures of living calcium. They have a greenish tint to their skin and stand eight feet tall with long forearms that end in powerful claws. They have a very developed sense of smell, able to track a single target across a few city blocks, and can sense if one of their own dies. Female Raxacoricofallapatorians can produce poisons within their bodies which they then use against their enemies. Known methods of delivery include a poisoned claw that can be fired like a dart and exhalation of poisoned breath.
The Slitheen are also able to 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. The compression ratio has its limits, however, so the disguises tend to be that of already big-bodied 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). The compression field also has the side effect of weakening their calcium structures, making them vulnerable to acetic acid, which reacts explosively - and fatally - with their bodies.