[4][need quotation to verify] The individual species of Old World monkey are more closely related to each other than to apes or any other grouping, with a common ancestor around 14 million years ago.

The largest is the male mandrill, around 70 centimetres (28 in) in length, and weighing up to 50 kilograms (110 lb)[6] Old World monkeys have a variety of facial features; some have snouts, some are flat-nosed, and many exhibit coloration.

Old World monkeys are native to Africa and Asia today, inhabiting numerous environments: tropical rain forests, savannas, shrublands, and mountainous terrain.

Most are highly opportunistic, primarily eating fruit, but also consuming almost any food item available, such as flowers, leaves, bulbs and rhizomes, insects, snails, small mammals,[6] and garbage and handouts from humans.

Other distinctions include both a tubular ectotympanic (ear bone), and eight, not twelve, premolars in catarrhines, giving them a dental formula of: 2.1.2.32.1.2.3 Several Old World monkeys have anatomical oddities.

[10] The Old World monkeys are native to Africa and Asia today, inhabiting numerous environments: tropical rain forests, savannas, shrublands, and mountainous terrain.

Leaf monkeys are the most vegetarian, subsisting primarily on leaves, and eating only a small number of insects, while the other species are highly opportunistic, primarily eating fruit, but also consuming almost any food items available, such as flowers, leaves, bulbs and rhizomes, insects, snails, and even small vertebrates.

[6] The Barbary macaque's diet consists mostly of leaves and roots, though it will also eat insects and uses cedar trees as a water source.

In most species, daughters remain with their mothers for life, so that the basic social group among Old World monkeys is a matrilineal troop.