Primates are usually classified as faunivores, frugivores, or folivores. Recent field studies have shown that plant exudates, mainly gums, are also a significant component of primate diets. Few other vertebrates consume gum. One group of primate gummivores, including the lemur Phaner, the galagine Euoticus, and especially the marmosets Cebuella and Cat-lithrix, has dental adaptations allowing its members to gouge or scrape trees to stimulate the flow of gum. Members of this group also have modifications of the nails which facilitate the postural repertoire required to harvest gums on large-diameter trunks. A second group, including Galago spp., Perodicticus potto, tamarins (Saguinus spp.), baboons (Papio spp.), vervets (Cercopithecus aethiops), and patas (Erythrocebus patas), is less morphologically specialized. These primates eat gums exuded in response to insect and/or mechanical damage. Gums are complex polysaccharides which, like foliage, require fermentation for digestion. At least some gummivorous primates show intestinal specializations for fermentation. Gums may also be an important source of minerals, especially calcium. Like folivores, gum feeders have the capacity to avoid secondary compounds found in gum, such as tannins and phenols. Gums are found in small, discrete patches. Each gum site can generally accommodate only one individual at a time, but the site may be rapidly renewed. One tree may contain many gum sites. Consequently, the foraging and digestive challenges entailed by gummivory involve a unique combination of features shows by faunivory, frugivory, and folivory. Understanding the biological bases of gummivory will be of value in interpreting the anatomy and modeling the behavior of early primates.