The order Testudines is composed of shelled reptiles with solid, anapsid skulls. It is the most primitive order of living reptiles, and is apparently close to the ancestral reptilian lineage (as indicated by its unspecialized skull). Living turtles are found on all continents except Antarctica. The marine species occur predominantly in tropical waters of the Atlantic, Pacific, and Indian oceans, but several also range far poleward in these waters. In maximum size, adult turtles range from less than 10 cm (Homopus signatus) to nearly 300 cm and over 900 kg (Dermochelys coriacea).

The most exceptional feature of the turtle is its shell; this extremely conservative character has remained little changed for about 200 million years. The shell is divided into two halves: an upper part, the carapace, and lower part, the plastron. The two parts are joined on each side by a bridge.

The typical carapace usually consists of about 50 bones (Carapacial bones of turtles). The nuchal is the most anterior bone along the midline, behind it are eight neurals (absent in some chelid turtles), two suprapygals, and a pygal, in that order. Occasionally a preneural may be found between the nuchal and the 1st neural, and the total number of neural bones may vary. The neurals are attached to the neural arches of the dorsal vertebrae, but the other bones of the series are free from the vertebrae. On each side of the neurals are usually eight costal bones; in some species a precostal is also present. Outside the costals and extending along each side from the nuchal to the pygal is a series of about 11 peripherals. Each carapacial bone articulates with the adjacent bones along a suture.

Members of the tortoise genus Kinixys have a movable hinge in their carapaces that is located between the 4th and 5th costal bones and 7th and 8th peripheral bones, which allows the posterior part of the carapace to be lowered over the hind limbs and tail, thus affording protection of these parts.

The forepart of the typical plastron is composed of a median bone, the entoplastron (absent in Kinosternidae), surrounded anteriorly by two epiplastra and posteriorly by two hyoplastra (Plastral bones of turtles). Behind these are a pair each of hypoplastra and xiphiplastra. In some primitive species a pair of mesoplastra occurs between the hyoplastra and hypoplastra. Between the forelimbs and the hind limbs the hyoplastra and hypoplastra articulate with the 3rd to 7th peripherals. The forelimb emerges from the axillary notch, the hindlimb from the inguinal notch. Just behind the axillary notches the axillary buttresses solidly attach the hyoplastra to the 1st costals, and in front of the inguinal notches the inguinal buttresses solidly attach the hypoplastra to the 5th costals. In some turtles, such as Terrapene, there is no bridge connecting the carapace to the plastron, and in others, such as Claudius, the bridge may be narrow or poorly buttressed.

In the families Dermochelyidae and Trionychidae (Carapacial bones of Apalone; Plastral bones of Apalone) and in the tortoise Malacochersus tornieri (M. tornieri; carapacial bones; M. tornieri; plastral bones), there has been a reduction in the size and/or number of bones in the shell, so the structures of their carapaces and plastra are quite aberrant.