The marmosets are the genus Callithrix of New World monkeys. One species not classified in this genus also has common names including the word "marmoset", the Goeldi's Marmoset (Callimico goeldii). This article deals only with the 18 species currently classified in Callithrix.

Most marmosets are about 20 cm in length. Relative to other monkeys, they show a number of apparently primitive features: they have claws rather than nails, and tactile hairs on their wrists. They lack wisdom teeth, and their brain layout seems to be relatively primitive. Their body temperature is unusually variable, showing as much as 4 Celsius degrees (7 Fahrenheit degrees) change in a day.

Marmosets are highly active, living in the upper canopy of forest trees, and feeding on insects, fruit and leaves. They have long lower incisors, which enable them to chew holes in the trunks or branches of trees so as to harvest the gum inside; some species are specialised feeders on gum.

Marmosets live in small family groups consisting of a mated pair and their offspring, thus making marmosets (along with gibbons) some of the few monogamous primates. They are thought to be territorial. In most species, twins are usually born, though triplets are not unknown. A high degree of paternal care is shown, and the male usually carries the infants on his back as the family group moves around.