Reptiles are tetrapods and amniotes, animals whose embryos are surrounded by an amniotic membrane, and members of the class Sauropsida. Today they are represented by four surviving orders:

• Crocodilia (crocodiles, gharials, caimans and alligators);: 23 species
• Sphenodontia (tuataras from New Zealand);: 2 species
• Squamata (lizards, snakes and amphisbaenids ("worm-lizards"););: approximately 7,900 species
• Testudines (turtles and tortoises);: approximately 300 species

Modern reptiles inhabit every continent except for Antarctica, although their main distribution comprises the tropics and subtropics. Though all cellular metabolism produces some heat, most modern species of reptiles do not generate enough to maintain a constant body temperature and are thus referred to as "cold-blooded" or ectothermic (the Leatherback Sea Turtle might be an exception, see also gigantothermy);. Instead, they rely on gathering and losing heat from the environment to regulate their internal temperature, e.g, by moving between sun and shade, or by preferential circulation — moving warmed blood into the body core, while pushing cool blood to the periphery. In their natural habitats, most species are adept at this, and can usually maintain core body temperatures within a fairly narrow range. Reptiles are thick-skinned; unlike amphibians, they do not need to absorb water. While this lack of adequate internal heating imposes costs relative to temperature regulation through behavior, it also provides a large benefit by allowing reptiles to survive on much less food than comparably-sized mammals and birds, who burn much of their food for warmth. While warm-blooded animals move faster in general, an attacking lizard, snake or crocodile moves very quickly.