The Rhesus Macaque (often called the Rhesus monkey) is one of the best known species of Old World monkeys. It is a typical macaque.
Native to India, the Rhesus monkey is well known because it is relatively easily kept in captivity and has therefore been used extensively in medical and biological research. It has given its name to the Rhesus factor, one of the elements of a person's blood group. Rhesus monkeys were also used in the well-known experiments on maternal deprivation carried out in the 1950s by comparative psychologist Harry F. Harlow.