- Crocodilia (crocodiles, gavials, caimans, and alligators): 23 species
- Sphenodontia (tuataras from New Zealand): 2 species
- Squamata (lizards, snakes, and worm lizards): approximately 7,900 species
- Testudines (turtles and tortoises): approximately 300 species
Unlike amphibians, reptiles do not have an aquatic larval stage. As a rule, reptiles are oviparous (egg-laying), although certain species of squamates are capable of giving live birth. This is achieved by either ovoviviparity (egg retention) or viviparity (birth of offspring without the development of calcified eggs). Many of the viviparous species feed their fetuses through various forms of placenta analogous to those of mammals, with some providing initial care for their hatchlings. Extant reptiles range in size from a tiny gecko, Sphaerodactylus ariasae, can grow up to 1.7 cm (0.6 in) to the saltwater crocodile, Crocodylus porosus, which may reach 6 m in length and weigh over 1,000 kg.
The study of reptiles and amphibians is called herpetology.
Herpetology is the branch of zoology concerned with the study of amphibians (including frogs, toads, salamanders, newts, and gymnophiona) and reptiles(including snakes, lizards, amphisbaenids, turtles, terrapins, tortoises, crocodilians, and the tuataras). Batrachology is a further subdiscipline of herpetology concerned with the study of amphibians alone.
Herpetology is concerned with poikilothermic, ectothermic tetrapods. Under this definition "herps" (or sometimes "herptiles" or "herpetofauna") exclude fish, but it is not uncommon for herpetological and ichthyological scientific societies to "team up", publishing joint journals and holding conferences in order to foster the exchange of ideas between the fields. One of the most prestigious organizations, the American Society of Ichthyologists and Herpetologists, is an example of this. Many herpetological societies exist today, having been formed to promote interest in reptiles and amphibians both captive and wild.
Herpetology offers benefits to humanity in the study of the role of amphibians and reptiles in global ecology, especially because amphibians are often very sensitive to environmental changes, offering a visible warning to humans that significant changes are taking place. Some toxins and venoms produced by reptiles and amphibians are useful in human medicine. Currently, some snake venom has been used to create anti-coagulants that work to treat stroke victims and heart attack cases.
The word "herpetology" is from Greek: ἑρπήτόν, herpeton, "creeping animal" and -λογία, -logia. People with an avid interest in herpetology and who keep different reptiles or amphibians often refer to themselves as "herpers".
"Herp" is a vernacular term for reptiles and amphibians. It is derived from the old term "herpetile", with roots back to Linnaeus' classification of animals, in which he grouped reptiles and amphibians together in the same class. There are over 6700 species of amphibians and over 9000 species of reptiles. In spite of its modern taxonomic irrelevance, the term has persisted, particularly in the names of herpetology, the scientific study of reptiles and amphibians, and herpetoculture, the captive care and breeding of reptiles and amphibians.