Living reptiles are classified in four orders. The turtles, order Chelonia, have a protective bony shell, usually covered with horny plates. They are mostly aquatic in habits although some (see tortoise) are adapted to land. They are the oldest living reptiles, having existed nearly unchanged since the Triassic period. Members of the order Crocodilia, which includes alligators, caimans, crocodiles, and gavials, are large, carnivorous reptiles of tropical and subtropical swamps and rivers. They constitute the only remaining order of the great reptilian subclass Archosauria, or ruling reptiles, which includes the extinct dinosaurs. The order Squamata includes the lizards (suborder Sauria) and snakes (suborder Serpentes). Nearly all members of this large and successful modern order are terrestrial. The order Rhynchocephalia has a single living member, the tuatara, a lizardlike reptile of New Zealand.

Reptiles first evolved from amphibians about 250 million years ago in the Carboniferous period and were dominant in the world's fauna during the Mesozoic era, sometimes called the Age of Reptiles. The dinosaurs, the marine Ichthyosaurus and Plesiosaurus, and the flying pterosaurs reached the peak of their development and distribution in the later part of this era (late Cretaceous period). Mammallike reptiles appeared very early in reptilian history and by the Triassic period had given rise to mammals. Bird ancestors arose from precursors of the dinosaurs; the first known birds lived in the Jurassic. The only reptiles that survived into the Cenozoic era belonged to the presently living orders. The approximately 6,000 living reptile species represent a very small fraction of this once vast class.