A tiny New World monkey (family Callitrichidae) from the Atlantic Forest of northeastern Brazil. Adults weigh only 250–350 g with a body length of 18–25 cm and a 29–35 cm banded tail. They sport distinctive white ear tufts, a white forehead blaze, and agouti-banded grayish-brown fur. Their lower incisors are specialized for gouging tree bark to access gum—a major dietary component alongside insects, fruits, and nectar. They typically give birth to dizygotic twins, and the entire group assists in infant carrying (cooperative breeding)—the father and older siblings do most of the carrying. Social groups consist of 3–15 individuals. They are diurnal and arboreal. Gestation lasts ~144 days. Common marmosets have become a premier non-human primate model in neuroscience, aging, infectious disease, and reproductive biology due to their small size, fast reproduction, and tractability. Transgenic marmosets have been successfully produced. Their genome is fully sequenced (2n=46). Lifespan: 10–16 years in captivity. IUCN: Least Concern.