Monkeys or simians are basal Haplorhini as sister of the Tarsiiformes. It consists of the Catarrhini and Platyrrhini (New World monkeys), and other extinct groups. The Catarrhini contains the Cercopithecoidea (Old World monkeys) and the Hominoidea (apes). Many monkey species are tree-dwelling (arboreal), although there are species that live primarily on the ground, such as baboons. Most species are also active during the day (diurnal). Monkeys are generally considered to be intelligent, particularly Catarrhini.
Simians and tarsiers emerged within haplorrhines some 60 million years ago. New World monkeys and catarrhine monkeys emerged within the simians some 35 million years ago. Old World monkeys and Hominoidea emerged within the catarrhine monkeys some 25 million years ago. Extinct basal simians such as Aegyptopithecus or Parapithecus[35-32 million years ago], eosimiidea and sometimes even the Catarrhini group are also considered monkeys by primatologists.
Lemurs, lorises, and galagos are not monkeys; instead they are strepsirrhine primates. Like monkeys, tarsiers are haplorhine primates; however, they are also not monkeys.
Apes emerged within the catarrhines with the Old World monkeys as a sister group, so cladistically they are monkeys as well. Traditionally apes were not considered monkeys, rendering this grouping paraphyletic.