Trachypithecus is a genus of Old World monkeys containing species known as lutungs, langurs, or leaf monkeys. Their range is much of Southeast Asia (northeast India, Vietnam, southern China, Borneo, Thailand, Java, and Bali).

The name "lutung" comes from the Sundanese language meaning "blackness", ultimately from Proto-Austronesian *luCuŋ (which originally referred to the Formosan rock macaque); it is preferred in one paper because the authors wanted the name langurs to only refer to monkeys in the genus Semnopithecus, although some "lutungs" are now "langurs" again. The scientific name of the genus comes from the Ancient Greek τραχύς (trakhús), meaning "rough", and πίθηκος (píthēkos), meaning "monkey".

Genetic analysis indicates that the ancestors of the modern species of lutung first differentiated from one another a little over 3 million years ago, during the late Pliocene. The various species alive today then diverged during the Pleistocene, presumably driven by habitat changes during the Ice Ages. The oldest fossils clearly identified as belonging to the genus date from the middle Pleistocene of Vietnam and Laos; later fossils are also known from Thailand, Java, and Sumatra. The closest living relatives of the lutungs are probably either the gray langurs or the surilis, although the exact relationships remain unclear, possibly due to hybridisation between these genera during the course of their recent evolutionary history.

As of 2005, the authors of Mammal Species of the World recognize the following Trachypithecus species:

Since then, the T. vetulus group (the purple-faced langur and the Nilgiri langur) have been moved the genus Semnopithecus based on DNA and other evidence.

In 2008, Roos et al. described the Malay Peninsula form of the silvery lutung as a separate subspecies and subsequently it has been elevated to a separate species within the T. cristatus group as the Selangor silvered langur, T. selangorensis. Roos et al. also elevated the West Javan Langur, Trachypithecus mauritius, and Annamese Langur, Trachypithecus margarita, to species status (formerly subspecies of T. auratus and T. germaini, respectively). In 2020, Roos et al. discovered a new species, Popa langur (T. popa), which is found only in Myanmar. Lastly, the White-headed langur (T. leucocephalus), previously thought to be a subspecies of the Francois langur (T. Francois) or Cat Ba langur (T. poliocephalus), is currently recognized as a distinct species by IUCN Red List assessors and the American Society of Mammalogists, based on a 2007 paper by Groves.

This leaves the current understanding of the genus Trachypithecus to be:

Lutungs have a rather slim build with a long tail. The fur color varies, depending on the species, from black and grey to orange yellow. Many species have skin designs and a brighter lower surface, the hair on the head is often compared to a hood. Their arms are very short in comparison to their hind legs, and their thumbs are also somewhat shorter than in other primates. The inner surfaces of the hands and feet are hairless so that their fur does not get caught when reaching into branches. These animals reach a length of 40 to 80 cm and a weight of 5 to 15 kg, with males being generally larger than females. A ridge over the eyes and other details, primarily related to the head, differentiate the lutungs from the surilis.