Pupfish are a group of small killifish belonging to ten genera of the family Cyprinodontidae of ray-finned fish. Pupfish are especially noted for being found in extreme and isolated situations. They are primarily found in North America, South America, and the Caribbean region, but Aphanius species are from southwestern Asia, northern Africa, and southern Europe. As of August 2006, 120 nominal species and 9 subspecies were known. Several pupfish species are extinct and most extant species are listed. In the U.S., the most well-known pupfish species may be the Devils Hole pupfish, native to Devils Hole on the Nevada side of Death Valley National Park. Since 1995 the Devils Hole pupfish has been in a nearly steady decline, where it was close to extinction at 35–68 fish in 2013.

A killifish is any of various oviparous (egg-laying) cyprinodontiform fish, including families Aplocheilidae, Cyprinodontidae, Fundulidae, Nothobranchiidae, Profundulidae and Valenciidae. All together, there are 1,270 species of killifish, the biggest family being Rivulidae, containing more than 320 species. As an adaptation to living in ephemeral waters, the eggs of most killifish can survive periods of partial dehydration. Many of the species rely on such a diapause, since the eggs would not survive more than a few weeks if entirely submerged in water. The adults of some species, such as Kryptolebias marmoratus, can additionally survive out of the water for several weeks. Most killies are small fish, from 2.5 to 5 centimetres, with the largest species growing to just under 15 centimetres (6 in).