Have you ever seen a monkey up close?

The traditional classification of simians including only monkeys and excluding apes is considered incomplete, forming a paraphyletic grouping. In a broader sense, based on cladistics, apes are also considered simians. According to Étienne Geoffroy in 1812, he grouped apes and monkeys together under the name Catarrhini. Platyrrhini emerged from monkeys due to migration from Afro-Arabia to South America, making apes closely related to the Cercopithecidae group of monkeys.

Most monkey species live in trees, though some like baboons prefer living on the ground. They are active mainly during the day. New World and catarrhine monkeys evolved about 35 million years ago. There are debates about including apes and humans in the term "monkey". The Old World monkey category can refer to Cercopithecoidea excluding apes or it can include apes under the Catarrhini group. Linnaeus classified monkeys in 1758 in a group called "Simia" with tarsiers excluding humans, now known as the Haplorhini group.

The term "monkey" may have originated from a German version of the Reynard the Fox fable dating back to around 1580. In the past, the English language did not clearly distinguish between "ape" and "monkey", as mentioned in the 1911 Encyclopædia Britannica. People often used the terms "monkey" and "ape" interchangeably in everyday conversation. As apes have been categorized within the monkey group alongside old-world monkeys, many characteristics that define monkeys are also found in apes. Monkeys come in various sizes, from the tiny pygmy marmoset to the large male mandrill. While New World monkeys typically have long tails, Old World monkeys may have non-prehensile tails or no visible tail.