Animal environments are classified as either aquatic (water), terrestrial (land), or amphibious (water and land). Aquatic animals require a watery habitat, but do not necessarily have to live entirely in water. This term can be applied to aquatic or sea mammals such as those in the order Cetacea (whales), which cannot survive on land, as well as four-footed mammals like the river otter (Lutra canadensis) and beavers (family Castoridae). It also includes aquatic birds that either swim, wade or dive on the water itself and live outside the water. These include the seabirds, such as gulls (family Laridae), pelicans (family Pelecanidae), and albatrosses (family Diomedeidae), and the anseriforms, such as ducks, swans, and geese (family Anatidae).

Amphibious and amphibiotic animals, like frogs (the order Anura), while they do require water, are separated into their own environmental classification. The majority of amphibians (class Amphibia) have an aquatic larval stage, like a tadpole, but then live as terrestrial adults, and may return to the water to mate.

Aquatic animals are often of special concern to conservationists because of the delicacy of their environments.