Reptiles are air-breathing, cold-blooded vertebrates that have scaly bodies rather than hair or feathers; most reptile species are egg-laying, though certain “squamates” — lizards, snakes and worm-lizards — give birth to live young. The earliest reptile is usually said to have been Hylonomus (a so-called "forest mouse"), which lived about 315 million years ago and resembled contemporary lizards. "Reptile" is an ambiguous category: it usually refers to lizards, snakes, turtles, alligators, and crocodiles, but to be genetically consistent should also include birds, since crocodilians are more closely related to birds than to lizards, snakes, or turtles. Turtles are so genetically distinct — they’re the sole surviving member of the Anapsid branch of the evolutionary tree — that many scientists recommend treating them as their own class (Chelonia) on an equal footing with birds, mammals, amphibians, fish, and reptiles.  Some scientists would also elevate crocodilians to the class level.

Globally, 422 species of reptiles, or five percent of the total 8,240 described species, were deemed endangered or vulnerable to extinction by IUCN's 2007 Red List. In the United States, 32 of about 347 species are at risk, or about nine percent of the total.