Old World monkeys are generally stocky in build and have fore limbs that are in most species shorter than hind limbs. Their skull is heavily ridged and they have a long rostrum. Almost all species are active during the day (diurnal) and are varied in their social behaviors. Many Old World monkey species form small to medium sized groups with complex social structure. The fur of Old World monkeys is often gray or brown in color although a few species have bright markings or more colorful fur. The texture of the fur is not silky nor is it woolly. The palms of the hands and soles of the feet in Old World monkeys are naked.
One distinguishing characteristic of Old World monkeys is that most species have tails. This distunguishes them from the apes, who do not have tails. Unlike New World monkeys, the tails of Old World monkeys are not prehensile.
There are a number of other characteristics that distinguish Old World monkeys from New World monkeys. Old World monkeys are comparitively larger than the New World monkeys. They have nostrils that are positioned close together and have a downward facing nose. Old World monkeys have two premolars that have sharp cusps. They also have opposable thumbs (similar to the apes) and they have nails on all fingers and toes.
New World monkeys have a falt nose (platyrrhine) and nostrils that are positioned far apart and open either side of the nose. They also have three premolars. New World monkeys have thumbs that are in line with their fingers and grip with a scissor-like motion. They do not have fingernails except for some species that have a nail on their largest toe.
Members of the Cercopithecinae have cheek pouches (also known as buccal sacs) that are used to store food. Since their diet is quite varied, Cercopithecinae have non-specialized molars and large incisors. They have simple stomachs. Many species of Cercopithecinae are terrestrial, although a few are arboreal. The facial muscles in Cercopithecinae are well developed and facial expressions are used to communicate social behavior.
Members of the Colobinae are folivorous and lack cheek pouches. They have complex stomachs.