Members of this order are distinctly recognized by the enclosure of the internal body organs in a hard shell. Only the head, neck, limbs and tail protrude. In America, species that are semiaquatic or fully aquatic are called turtles, whereas those species that primarily live on land are called tortoises. Prototurtles evolved more than 200 million years ago, during the Triassic period, prior to the existence of the dinosaurs. The turtle has thrived for 150 million years in its present form.

The order testudines is divided into two suborders. Cryptodira, the larger of the two, are sometimes called straight or hidden necked turtles. These species pull their necks straight back into their shells. The suborder Pleurodira, or side necked turtles, withdraw their heads by pulling their necks to the side. The neck can only be partially withdrawn into the carapace, or not at all, so it is bent laterally under the front edge of the carapace. Today, species of this suborder are found only in Australia, South America and Africa.

The turtle skull, unlike that of other reptiles, is considered primitive. It has a solid cranium with no apses or holes in the temporal region. The shape of the jaw varies from species to species. Turtles have no teeth, instead their jaw has sharp horned edges that function like scissors. The sense of sight and smell are important for obtaining food. Turtles have well-developed ears, however, they apparently cannot hear very well. The skin is tough and scaly, yet flexible. The limbs of turtles are shaped for their environment and style of locomotion. The front legs of aquatic species are flippers. Amphibious species have webbed feet used for both walking and swimming. The toes of some terrestrial species have fused into stumpy feet. In all of these, the limbs may or may not have claws.

The shell of testudines consists of two parts; the carapace, or upper section, and the plastron, or lower section. The two are joined by bony bridges on the sides of the shell. The protruding, movable parts of the turtles stick out through the gaps located at the front and back of the shell. Some species have a plastron that is hinged in several places, allowing it to flex and completely enclose the head, tail and limbs. Evolution has fostered a change in the composition and shape of the shell, depending upon the environment inhabited by the turtle. The carapace of the terrestrial species is dome shaped, for protection from predators. Aquatic species have low, streamlined shells that reduce water resistance when swimming. Soft shelled turtles have developed a shell with a flat round shape that is covered by tough leathery skin. This shell construction enables the species to hide beneath sand or mud at the bottom of lakes and ponds.

Sexing turtles is often difficult because sexual organs are within the cloaca. Some external characteristics, such as plastron shape and tail size, may be used to identify the sex, but not all characteristics apply to all species. Sexual maturity differs from species to species. Two to five years for smaller species and eight to ten years in larger species. After mating, the female deposits her eggs in the ground, in a very general kind of nest. Clutch size varies from 1 to 4 eggs for smaller species to over 100 eggs for larger species. Incubation periods range from two months to over one year. Through methods of sperm storage, the female can lay fertile eggs for up to four years without mating. Turtle eggs may be spherical or elongated with a flexible or inflexible shell.

Depending on the species, turtles may live to over 150 years of age. The species vary greatly in size, ranging from 5 inches to over 8 feet. Most tortoise are diurnal, active during the day time. Sea turtles are often active at dawn or dusk or completely nocturnal. The diet of turtles and tortoises may be herbivorous, omnivorous or carnivorous.