Orangutans are great apes native to the rainforests of Indonesia and Malaysia. They are now found only in parts of Borneo and Sumatra, but during the Pleistocene they ranged throughout Southeast Asia and South China. Classified in the genus Pongo, orangutans were originally considered to be one species. From 1996, they were divided into two species: the Bornean orangutan (P. pygmaeus, with three subspecies) and the Sumatran orangutan (P. abelii). In 2017, a third species, the Tapanuli orangutan (P. tapanuliensis), was definitively identified. The orangutans are the only surviving species of the subfamily Ponginae, which split from the other hominids (gorillas, chimpanzees, and humans) between 19.3 to 15.7 million years ago.

The most arboreal of the great apes, orangutans spend most of their time in trees. They have proportionally long arms and short legs, and have reddish-brown hair covering their bodies. Adult males weigh about 75 kg (165 lb), while females reach about 37 kg (82 lb). Adult males may develop distinctive cheek pads or flanges and make long calls that attract females and intimidate rivals; younger males do not and more resemble adult females. Orangutans are the most solitary of the great apes, social bonds occurring primarily between mothers and their dependent offspring, who remain together for the first two years. Fruit is the most important component of an orangutan's diet, but they will also eat vegetation, bark, honey, insects and bird eggs. They can live over 30 years, both in the wild and in captivity.