The small bony or horny plates forming the body covering of fish and reptiles. The wings of some insects, notably the Lepidoptera (butterflies and moths), are covered with tiny scales that are modified cuticular hairs.

In fish there are three types of scales. Placoid scales (denticles), characteristic of cartilaginous fish, are small and toothlike, with a projecting spine and a flattened base embedded in the skin. They are made of dentine, have a pulp cavity, and the spine is covered with a layer of enamel. Teeth are probably modified placoid scales. Cosmoid scales, characteristic of lungfish and coelacanths, have an outer layer of hard cosmin (similar to dentine) covered by modified enamel (ganoine) and inner layers of bone. The scale grows by adding to the inner layer only. In modern lungfish the scales are reduced to large bony plates. Ganoid scales are characteristic of primitive ray-finned fishes, such as sturgeons. They are similar to cosmoid scales but have a much thicker layer of ganoine and grow by the addition of material all round. The scales of modern teleost fish are reduced to thin bony plates.

In reptiles there are two types of scales: horny epidermal corneoscutes sometimes fused with underlying bony dermal osteoscutes. The feathers of birds are now thought to have evolved from the scales of their reptile ancestors.