Caves are extreme environments. Organisms that live underground are quite different from what we encounter in our sunlit surface world. This has led some animals to develop strikingly unique adaptations to a perpetually dark, nutrient-poor subterranean world.

Animals that use caves are called "troglobites," "stygobites," "trogloxenes" and "troglophiles." Troglobites and stygobites are limited to living their entire life in the cave environment. These animals are typically characterized by lack of pigmentation, eyes that are reduced or completely absent, and elongated appendages, antennae and hairs that better equip them for surviving in a world without light. Troglobites are terrestrial cave-adapted animals that occur only in caves or similar subterranean habitats. Examples of troglobites include blind spiders, albinistic millipedes and slender, ghostly salamanders.

Stygobites are aquatic cave-adapted organisms, and include eyeless catfish, shrimp, and crawfish. Trogloxenes are animals that live in caves (or similar subterranean environments) during part of their life cycles, but must also spend part of their life outside, on the surface. They typically use caves for shelter, but return to the surface to forage. The most notable trogloxenes are bats and cave crickets. Troglophiles are animals that may complete their entire life cycle within caves, but can survive equally well in surface habitats. Examples of troglophiles include some spiders, crickets, beetles and salamanders (tiger salamander featured at right).

Globally, caves and the animals they support have not been well-studied. However, we do know these ecosystems are sensitive to a variety of disturbances. While they are protected to some degree by their isolated nature, human-induced impacts from the surface are increasing in many areas. Some of the better-studied areas in the United States include the Edwards Plateau region of Texas and the Appalachian Mountains in the eastern U.S. Caves and the animals they support in the southwestern United States, in contrast, are very poorly-known.