They are found only in the tropical forests of southeastern Asia, including on Sumatra, Borneo, Java, and other islands as well as the mainland. They lack tails. Their forearms are remarkably long, and both forefeet and hindfeet are long and thin. The forefeet have a deep cleft between first and second digits. The pelage of these apes is usually black, gray, or brownish, often with white markings on hands, feet, and facial region.
Ischial callosities are present.
Some gibbons and siamangs 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. The skulls of hylobatids resemble those of hominids, with very short rostra, enlarged braincases, and large orbits that face forward. They lack cheek pouches and their stomach is not sacculated. Their teeth are similar to those of hominids.