All reptiles have scales or scutes that cover their skin, as this is a defining characteristic of the class Reptilia. Reptilian skin is horny and waterproof but lacks the thickness of mammalian skin. In some reptiles, the scales or scutes have a bony base that provides more protection.

Reptilian scales cannot detach from the skin, contrasting with the scales of fish. Scales form either plate-like or overlapping patterns. When shedding, reptiles discard their outermost layer of skin including the scales. This allows the animal to grow larger. All reptiles are tetrapods, meaning they either have four legs or are descendants from four-legged ancestors, which is the case with snakes and legless lizards.