There is much variation in size, coloration, and other details. Almost all forms have dusky brown or black facial markings around the eyes. The fur is short, fine, and velvety in most species. The tail is strongly prehensile. There is no pouch.
Most species are forest dwellers, but some occur on the Argentine pampas or in the Andes.
The genus ranges vertically from sea level to about 3,400 meters. Mouse opossums are nocturnal and usually arboreal, though some species are terrestrial. They are often found on banana plantations and among small trees and vine tangles. They build nests of leaves and twigs in trees or shelter in abandoned birds' nests. They have also been located in ground nests, in holes in hollow logs, and under rocks.