The Old World monkeys are found in S Asia, with a few species as far N as Japan and N China, and in all of Africa except the deserts. Most are arboreal, but a few, such as baboons and some macaque species, are ground dwellers. Some Old World monkeys lack tails; when a tail is present it may be long or short but is never prehensile (grasping). The nostrils are close together and tend to point downward. Many species have cheek pouches for holding food, and many have thick pads (called ischial callosities), on the buttocks. Their gestation period is five to nine months. Adult Old World monkeys have 32 teeth. The Old World monkeys, sometimes called true monkeys, are more closely related to the apes and humans than they are to the New World monkeys; the two monkey groups probably evolved separately from ancestral primates.

The Old World monkeys include the many species of macaque, widely distributed throughout Africa and Asia. The rhesus monkey, commonly used in laboratory experiments, is an Asian macaque. Related to the macaques are the baboons of Africa and SW Asia, as well as the mandrill and mangabey of Africa. The guerezas, or colobus monkeys (genus Colobus), are very large, long-tailed, leaf-eating African monkeys. Their Asian relatives, the langurs and leaf monkeys, include the sacred monkeys of India. The snub-nosed monkey of China and the proboscis monkey of Borneo are langurlike monkeys with peculiar snouts. The guenons (Cercopithecus) are a large group of long-legged, long-tailed, omnivorous monkeys found throughout sub-Saharan Africa. One very widespread guenon species is the green monkey, or vervet, with olive-brown fur.

The New World monkeys are found from S Mexico to central South America, except in the high mountains, and are classified into two families (Callatrichids and Cebids). The Callatrichids are very small, while the Cebids are similar in size to the Old World monkeys. They are all thoroughly arboreal and most have long, prehensile tails with which they can manipulate objects and hang from branches. In most the thumb is lacking. They have widely separated nostrils that tend to point outward; they lack cheek pouches and ischial callosities. Their gestation period is four to five months. Adults of most New World species have 36 teeth.

The New World monkeys include the marmosets and tamarins, small monkeys with claws that are classified in a family of their own, the Callithricidae. The rest of the New World monkeys are classified in the family Cebidae. They include the capuchin (genus Cebus), commonly seen in captivity, which has a partially prehensile tail. Prehensile tails are found in the spider monkey and woolly monkey as well as in the howler monkey, the largest member of the family, which has a voice that carries several miles. Smaller forms with nonprehensile tails are the squirrel monkey and titi, the nocturnal douroucouli, or owl monkey, the saki, and the ouakari.

Monkeys are classified in the phylum Chordata, subphylum Vertebrata, class Mammalia, order Primates, superfamilies Cercopithecoidea and Ceboidea.