Snakes are slender, legless, carnivorous reptiles of suborder snakes that can be distinguished from legless lizards because they have no eyelids or outer ears. Like all bulbs, snakes are endothermic, amniotic vertebrates covered in scales. Many species of snakes have more joints in their skulls than their ancestors, allowing them to swallow much larger prey through their movable jaws. To accommodate a narrow body, pairs of organs, such as kidneys, appear side by side, not side by side, and most have only one functional lung. Some species retain pelvic bands with a pair of residual claws on both sides of the cloaca.

Live snakes have been found on all continents except Antarctica and on most smaller lands. Exceptions include large islands such as Ireland, Iceland, Greenland, the Hawaiian Islands and New Zealand, as well as many small islands in the Atlantic and Central Pacific. In addition, sea snakes are common throughout the Indian Ocean and the Pacific Ocean. At present, there are more than 20 families, including about 500 genera and about 3400 species. They range in size from tiny snakes 10.4cm (4.1in) long to mesh Python 6.95m (22.8ft) long. The fossil species Titanoboa cerrejonensis is 12.8 meters (42 feet) long. It is believed that snakes may have evolved from caves or aquatic lizards, perhaps during the Jurassic period, with the earliest known fossils dating from 143 Ma to 167 Ma ago. The diversity of modern snakes occurs in the Paleocene (about 66 Ma to 56 Ma ago). The oldest description of a snake can be found on papyrus Brooklyn.

Most species are non-toxic, and toxic species use them primarily to kill and subdue prey, rather than to defend themselves. Some venoms are toxic enough to cause human suffering or death. Non venomous snakes either swallow prey or are killed by constriction.