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

• Crocodilia (crocodiles, caimans and alligators): 23 species
• Rhynchocephalia (tuataras from New Zealand): 2 species
• Squamata (lizards, snakes and amphisbaenids ("worm-lizards")): approximately 7,600 species
• Testudines (turtles): approximately 300 species

Reptiles are found on every continent except for Antarctica, although their main distribution comprises the tropics and subtropics. Though all cellular metabolism produces some heat, modern species of reptiles do not generate enough to maintain a constant body temperature and are thus referred to as "cold-blooded" (ectothermic). (See the Leatherback Sea Turtle for an exception: a reptile that elevates its body temperature well above that of its surroundings.) 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 maintain core body temperatures within a fairly narrow range, comparable to that of mammals and birds, the two surviving groups of "warm-blooded" animals. 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. A herpetologist is a zoologist who studies reptiles and amphibians.

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