Reptiles, the class Reptilia, comprise today's turtles, crocodilians, snakes, lizards and tuatara, their extinct relatives, and some of the extinct ancestors of mammals.

The earliest known reptiles originated around 315 million years ago during the Carboniferous Period, having evolved from advanced reptile-like amphibians that became increasingly adapted to life on dry land. Some early examples include the lizard-like Hylonomus, Casineria and possibly Westlothiana. In addition to the living reptiles, there are many diverse groups that are now extinct, in some cases due to mass extinction events.

Reptiles are tetrapod vertebrates, creatures that either have four limbs or, like snakes, being descended from four-limbed ancestors. Unlike amphibians, reptiles do not have an aquatic larval stage. Most reptiles are oviparous [animals that lay eggs, with little or no other embryonic development within the mother], although several species of are viviparous [development of the embryo inside the body of the mother contained in a placenta rather than an eggshell, eventually leading to live birth], as were some extinct aquatic life forms. Reptile eggs are surrounded by membranes for protection and transport, which adapt them to reproduction on dry land. Extant reptiles range in size from a tiny gecko, which can grow to 17 mm, to the saltwater crocodile, which may reach 6 m in length and weigh over 1,000 kg.