Gills are a respiratory structure found on various types of aquatic animals, including all known fish. They allow the animal to extract oxygen from the water and diffuse carbon dioxide out into it, allowing them to breathe without coming up to the surface for air, as marine mammals must do.

The term gills are most often used as synecdoche for the gill slits, parallel slits leading to the pharynx where the actual gills are located internally. In cartilaginous fish (Class: Chondrichthyes) and jawless fish (Infraphylum: Agnatha), they have several gill slits or spiracles; in bony fish (Superclass: Osteichthyes), a single pair of a bony gill cover called an operculum.

Alternatively, the gills maybe located outside of the pharynx and exposed to the environment, a trait most commonly seen in amphibians. Most species of salamanders possess external gill during their larval stages, and are even retained in those that don't metamorphose into an air-breathing form such as axolotls and olms.

Anthropomorphic characters may have gills located elsewhere instead, such as on the sides of the torso, head, or arms.