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The class Reptilia consists of over 7,000 living species which belong to one of four main reptile groups. Those four main groups are crocodilia, sphenodontia, squamata, and testudines. Despite all reptiles being air-breathing, cold-blooded vertebrates; each group of reptile is uniquely different.

The order (or group) crocodilia consists of crocodiles, alligators, gharials and caimans. These large reptiles are typically 6 to 13 feet in length, with some species growing much larger. Reptiles within this group live mostly in warm tropical parts of the world, although alligators can be found in warmer southern regions of the United States and China. Most of a crocodilian's life is spent in the water, using their strong flattened tails to swim. Their skin is covered with scales which are non-overlapping and composed of keratin. The eyes and nostrils are located on the top of their heads. This placement allows crocodilia to see and breathe while their body remains submerged, this enables them to stealth though water. Unlike most other reptiles, crocodilians have a secondary bony palate which enables them to breathe when partially submerged.

Sphenodontia consists of one living genus, the tuatara. This lizard-like reptile is found only on a few islands off the coast of New Zealand. They typically range from about 18 to 24 inches, and are slow in growth and development. They don't reach sexual maturity until they are about 15 to 20 years, and females are only ready to mate about once every two to five years. Unlike other reptiles, the male tuatara does not have a penis and passes the sperm from the cloaca. During the first six months, a "third eye" is visible under the young tuatara's skin and eventually becomes covered in scales. This "third eye" allows the young tuatara to get Vitamin D from the UV rays that the eye soaks up. The tuatara continue to grow up to 35 years old, and may even live for over 100 years. This species is sometimes referred to as living fossils as they are the survivors of a group of reptiles that emerged over 225 million years ago.

The largest order of reptiles is the squamata which consists of lizards, snakes, and amphisbaenids (worm lizards). Squamata are distinguished by their skin which bears horny scales or shields. Lizards typically have long, slender bodies that taper to a narrow, pointed tail. Most lizards have four legs with clawed toes, the worm lizards however have no limbs at all. Lizards usually range between 4 to 8 inches in length,

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