The monito del monte (Dromiciops gliroides), or colocolo opossum, is a diminutive species of marsupial native only to south-western South America (Argentina and Chile). It is the only extant species in the ancient order Microbiotheria, and the sole New World representative of the superorder Australidelphia, being more closely related to Australian marsupials than to other American marsupials. The species is nocturnal and arboreal, and lives in thickets of South American mountain bamboo in the Valdivian temperate forests of the southern Andes, aided by its partially prehensile tail. It consumes an omnivorous diet based on insects and fruit.

Dromiciops gliroides is the sole extant member of the order Microbiotheria. It was first described by British zoologist Oldfield Thomas in 1894. The generic name Dromiciops is based on the resemblance of the monito del monte to the eastern pygmy possum (Cercartetus nanus), one of the synonyms of which is Dromicia nana. The specific name gliroides is a combination of the Latin glis, gliris ("dormouse", more generally "rodent") and Greek oides ("similar to"). The name australis in a synonym (D. australis) refers to the southern distribution of the animal. The common name monito del monte is Spanish for "little monkey of the bush".

In his 1943 Mammals of Chile, American zoologist Wilfred Hudson Osgood identified two subspecies of the monito del monte:

South American marsupials have long been suspected to be ancestral to those of Australia, consistent with the fact that the two continents were connected via Antarctica in the early Cenozoic. Australia's earliest known marsupial is Djarthia, a primitive mouse-like animal that lived in the early Eocene about 55 million years ago (mya). Djarthia had been identified as the earliest known australidelphian, and this research suggested that the monito del monte was the last of a clade that included Djarthia. This relationship suggests that the ancestors of the monito del monte might have reached South America by back-migration from Australia. The time of divergence between the monito del monte and Australian marsupials was estimated to have been 46 mya.

Dromiciops is thought to have evolved from members of the genus Microbiotherium, known from the early Miocene of South America, with some authors considering the genera indistinguishable. All other genera, like Pachybiotherium, had become extinct by the late Miocene.

However, in 2010, analysis of retrotransposon insertion sites in the nuclear DNA of a variety of marsupials, while confirming the placement of the monito del monte in Australidelphia, also clarified that its lineage is the most basal of that superorder. The study further confirmed that the most basal of all marsupial orders are the other two South American lineages (Didelphimorphia and Paucituberculata, with the former probably branching first). This conclusion indicates that Australidelphia arose in South America (along with the ancestors of all other living marsupials), and probably reached Australia in a single dispersal event after Microbiotheria split off. Fossils of another Eocene australidelphian, the microbiotherian Woodburnodon casei, have been described from the Antarctic Peninsula, and fossils of a related early Eocene woodburnodontid have been found in Patagonia.

Monitos del monte live in the dense forests of highland Argentina and Chile, mainly in trees, where they construct spherical nests of water-resistant colihue leaves. These leaves are then lined with moss or grass, and placed in well-protected areas of the tree, such as underbrush, tree cavities, or fallen timber. The nests are sometimes covered with gray moss as a form of camouflage. These nests provide the monito del monte with some protection from cold, both when it is active and when it hibernates.

Fragmentation of Valdivian temperate rainforests into non-contiguous areas is known to reduce the abundance of monitos del monte in a given area, but has little or no impact on whether it occurs in an area or not.