Old World monkeys are primates in the family Cercopithecidae (). Twenty-four genera and 138 species are recognized, making it the largest primate family. Old World monkey genera include baboons (genus Papio), red colobus (genus Piliocolobus), and macaques (genus Macaca). Common names for other Old World monkeys include the talapoin, guenon, colobus, douc (douc langur, genus Pygathrix), vervet, gelada, mangabey (a group of genera), langur, mandrill, drill, surili (Presbytis), patas, and proboscis monkey.
Phylogenetically, they are more closely related to apes than to New World monkeys, with the Old World monkeys and apes diverging from a common ancestor between 25 million and 30 million years ago. This clade, containing the Old World monkeys and the apes, diverged from a common ancestor with the New World monkeys around 45 to 55 million years ago. 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.