A primate is a mammal of the order Primates. Primates arose from ancestors that lived in the trees of tropical forests and many primate characteristics are thought to represent adaptations to life in this challenging three-dimensional environment. Most primate species remain at least partly arboreal.

With the exception of humans, who inhabit every continent, most primates now live in tropical or subtropical regions of the Americas, Africa and Asia. They range in size from Madame Berthe's mouse lemur, which weighs only 30 grams, to the eastern gorilla, weighing over 200 kilograms. Based on fossil evidence, the earliest known true primates, represented by the genus Teilhardina, date to 55.8 million years old. An early close primate relative known from abundant remains is the Late Paleocene Plesiadapis, about 55-58 million years old. Molecular clock studies suggest that the primate branch may be even older, originating near the Cretaceous-Paleocene boundary around 63-74 million years ago.

Considered generalist mammals, primates exhibit a wide range of characteristics. Some primates (including some great apes and baboons) are primarily terrestrial rather than arboreal, but all species possess adaptations for climbing trees. Locomotion techniques used include leaping from tree to tree, walking on two or four limbs, knuckle-walking, and swinging between branches of trees.

Primates are characterized by large brains relative to other mammals, as well as an increased reliance on stereoscopic vision at the expense of smell, the dominant sensory system in most mammals. These features are more developed in monkeys and apes and noticeably less so in lorises and lemurs. Three-color vision has developed in some primates. Most also have opposable thumbs and some have prehensile tails. Many species are sexually dimorphic. Differences include body mass, canine tooth size, and coloration. Primates also have slower rates of development than other similarly sized mammals and reach maturity later, but have longer lifespans. Depending on the species, adults may live in solitude, in mated pairs, or in groups of up to hundreds of members.