Little Monkey enjoying with his Father and Mother

Little monkey enjoying with his family: Many monkey species are tree-dwelling (arboreal), although there are species that live primarily on the ground, such as baboons. Most species are mainly active during the day (diurnal). Monkeys are generally considered to be intelligent, especially the Old World monkeys.

Simians and tarsiers, the two members of the suborder Haplorhini, emerged some 60 million years ago. New World monkeys and catarrhine monkeys emerged within the simians some 35 million years ago. Old World monkeys and apes emerged within the catarrhine monkeys some 25 million years ago. Extinct basal simians such as Aegyptopithecus or Parapithecus [35–32 million years ago] are also considered monkeys by primatologists.[3][4][5][6][7]

Lemurs, lorises, and galagos are not monkeys; instead they are strepsirrhine primates (suborder Strepsirrhini). The simians' sister group, the tarsiers are also haplorhine primates; however, they are also not monkeys.

Apes emerged within "monkeys" as sister of the Cercopithecidae in the Catarrhini, so cladistically they are monkeys as well. There has been resistance to directly designate apes (and thus humans) as monkeys, so "Old World monkey" may be taken to mean either the Cercopithecoidea (not including apes) or the Catarrhini (including apes). That apes are monkeys was already realized by Georges-Louis Leclerc, Comte de Buffon in the 18th century.

Monkeys, including apes, can be distinguished from other primates by having only two pectoral nipples, a pendulous penis, and a lack of sensory whiskers.