Gibbons are the small apes that are grouped in the family Hylobatidae. The family is divided into four genera based on their diploid chromosome number: Hylobates (44), Hoolock (38), Nomascus (52), and Symphalangus (50). They occur in tropical and subtropical rainforests from northeast India to Indonesia and north to southern China, including the islands of Sumatra, Borneo and Java. The extinct Bunopithecus sericus is an extinct gibbon or gibbon-like ape which, until recently, was thought to be closely related to the Hoolock gibbons.

Also called the lesser apes, gibbons differ from great apes (chimpanzees, gorillas, orangutans and humans) in being smaller, pair-bonded, in not making nests, and in certain anatomical details in which they superficially more closely resemble monkeys than the great apes do. Gibbons are masters of their primary mode of locomotion, brachiation, swinging from branch to branch distances of up to 15 m (50 ft), at speeds as much as 56 km/h (35 mph). They can also make leaps of up to 8 m (27 ft), and walk bipedally with their arms raised for balance.

One unique aspect of gibbon physiology is that the wrist is composed of a ball and socket joint, allowing for biaxial movement. This greatly reduces the amount of energy needed in the upper arm and torso, while also reducing stress on the shoulder joint. They also have long hands and feet, with a deep cleft between the first and second digits of their hands. Their fur is usually black, gray, or brownish, often with white markings on hands, feet, and face. Some species have an enlarged throat sac, which inflates and serves as a resonating chamber when the animals call. This structure is enormous in a few species, equalling the size of the animal's head.

Gibbon skulls resemble those of the great apes, with very short rostra, enlarged braincases, and large orbits that face forward. Gibbons have the typical nose of catarrhine primates with nostrils that are close together and face forward and slightly downward. They lack cheek pouches and their stomach is not sacculated. Their teeth also are similar to the great apes, with molars that are bunodont and lack lophs. The upper molars usually have a cingulum, which is sometimes large. The canines are prominent but not sexually dimorphic.
Strongly territorial, gibbons defend their boundaries with vigorous visual and vocal displays. The vocal element, which can often be heard for long distances, consists of a duet between the mated pair, the young animals sometimes joining in. This eerie song can make them an easy find for poachers who engage in the illegal wildlife trade and in sales of body parts for use in traditional medicine. Most species are threatened or endangered, most importantly from degradation or loss of their forest habitat.

Gibbon species include the Siamang, the White-handed or Lar Gibbon, and the Hoolock gibbons. The Siamang, which is the largest of the 13 species, is distinguished by having two fingers on each hand stuck together, hence the generic and species names Symphalangus and syndactylus.