Reptiles first appeared on earth about 260 million years ago during the Upper Carboniferious period. The greatest evolutionary development of reptiles was during the Mesozoic era, 240 to 260 million years ago. It was during this time that reptiles flew, were mammal-like, fish-like, had paddle-like feet or had plate-like teeth. There were over twenty orders of reptiles as opposed to the four orders we see today.

Present day reptiles have one of three body forms. The most primitive is a long body with a long tail, indicative of crocodilians and lizards. A second form, common in snakes, consists of a long body with a short tail that is not clearly differentiated. The third form, common to turtles, consists of a short tail and a body enclosed in a shell.

Reptilian respiration is entirely pulmonary, except in certain turtles. The lungs are simple sac-like organs with walls that fold inward into ridges with many small compartments that increase the respiratory surface. All reptiles have three-chambered hearts, except for crocodilians, which have a fourth chamber. Due to these more simplistic respiratory and circulatory systems, reptiles have only enough oxygen to supply their tissues and maintain metabolic processes and so their body temperature can only rise a few degrees above their surrounding temperatures. This makes reptiles ectothermic, dependent on external heat sources such as the sun to raise their body temperature.

The skin of reptiles has a horny surface layer that forms a covering over scales, which are continuous along the entire body. The epidermis is a uniform structure of dead cells that must be shed periodically to allow the body to grow. Reptilian teeth are found on the bones of the upper or lower jaw and loss and replacement of teeth occur throughout life. The salivary glands of reptiles are well developed and help in lubrication and digestion of food. In snakes, and some lizards, the tongue is a highly developed and mobile sensory organ. The Jacobson's organ, in the roof of the mouth, is an important chemical sense organ that helps in identifying odors. Vision is one of the most important senses to reptiles and is used in identifying prey, mates, or rivals by sight. All reptiles, except for snakes, have a middle ear eustacian tube, ossicles and tympanum. The reptilian ear primarily detects tones of low frequency. The diet of reptiles may be carnivorous, herbivorous, insectivorous, ophiophagous or piscivorous.

Sexual dimorphism is present to a limited degree in snakes, crocodilians and turtles. It is more noticeable in lizards. Reproduction in reptiles is either oviparous or ovovivoparous. Reptile reproduction was one of the greatest evolutionary changes. The development of the yolk-rich shell-enclosed egg allowed embroyonic growth to shift to land. Eggs with shells require internal fertilization. Most male reptiles have copulatory organs to transfer sperm in the cloaca of the female. Most reptiles lay their eggs on the ground or in scooped-out hollows, which are later filled. The selection of the nesting site by the female is important, for it is necessary to insure both moisture and warmth from the sun. Reptiles rarely incubate their eggs, and only in a few species will the female protect them until they hatch.