Order Crocodilia (crocodiles and alligators): 23 species
Order Rhynchocephalia (tuataras from New Zealand): 2 species
Order Squamata (lizards and snakes): approximately 7,600 species
Order Testudines (turtles): approximately 300 species

Reptiles are found on all continents except for Antarctica, although their main distribution comprises the tropics and subtropics. Reptiles don't have a constant body temperature. They are only able to a limited extent to actively regulate their body temperature, which is largely dependent on the environmental temperature. Most reptile species are carnivorous and oviparous (egg-laying). Some species are ovoviviparous, and a few species are truly viviparous.

However, note the below described taxonomy issues; mammals and birds are all descendants of reptiles.

Classification of reptiles

Reptiles classically included all the amniotes except birds and mammals. Thus reptiles were defined as the set of animals that includes crocodiles, alligators, tuataras, lizards, snakes, and turtles, grouped together as the class Reptilia. This is still the usual definition of the term.

However, in recent years many taxonomists have begun to insist that taxa should be monophyletic, that is, groups should include all descendants of a particular form. The reptiles as defined above would be paraphyletic, since they exclude both birds and mammals, although these also developed from the original reptile. Colin Tudge writes:

Mammals are a clade, and therefore the cladists are happy to acknowledge the traditional taxon Mammalia; and birds, too, are a clade, universally ascribed to the formal taxon Aves. Mammalia and Aves are, in fact, subclades within the grand clade of the Amniota. But the traditional class reptila is not a clade. It is just a section of the clade Amniota: the section that is left after the Mammalia and Aves have been hived off. It cannot be defined by synamorphies, as is the proper way. It is instead defined by a combination of the features it has and the features it lacks: reptiles are the amniotes that lack fur or feathers. At best, the cladists suggest, we could say that the traditional Reptila are 'non-avian, non-mammalian amniotes'. (Tudge, p.85)

Some cladists thus redefine Reptilia as a monophyletic group, including both the classic reptiles as well as the birds and perhaps the mammals (depending on ideas about their relationships). Others abandon it as a formal taxon altogether, dividing it into several different classes. However, other biologists believe that the common characters of the standard four orders are more important than the exact relationships, or feel that redefining the Reptilia to include birds and mammals would be a confusing break with tradition. A number of biologists have adopted a compromise system, marking paraphyletic groups with an asterisk, e.g. class Reptilia*. Colin Tudge notes other uses of this compromise system:

By the same token, the traditional clas Amphibia becomes Amphibia*, because some ancient amphibian or other gave rise to all the amniotes; and the phylum Crustecea becomes Crustacea*, because it may have given rise to the insects and myriapods (centipedes and millipedes.) if we believe, as some (but not all) zoologists do, that myripaods gave rise to insects, then they should be called Myriapoda*....by this convention Reptilla without an asterisk is synonmous with Amniota, and incliudes birds and mammals, where as Reptila* means non-avian, non-mammalian amniotes. (Tudge, p.85)