New World monkey species of the genera Callithrix, Cebuella, Callibella, and Mico. All four genera are part of the biological family Callitrichidae. The term marmoset is also used in reference to the Goeldi's Monkey, Callimico goeldii, which is closely related.

Most marmosets are about 20 centimetres (8 in) long. Relative to other monkeys, they show some 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, changing by up to 4 °C (7 °F) in a day.