Marsupials are an infraclass of mammals living primarily in Australasia and the Americas. A distinctive characteristic, common to most species, is that the young are carried in a pouch. Well-known marsupials include kangaroos, wallabies, and the koala, possums, opossums, wombats and Tasmanian devil.

Marsupials represent the life-form group originating with the last common ancestor of extant metatherians [Metatheria is a group of animals that includes all mammals more closely related to marsupials than to placentals]. Like other mammals they are characterized by giving birth to relatively undeveloped young, often residing in a pouch with the mother for a certain time after birth.

The ancestors of marsupials are thought to have probably split from the placental mammals during the mid-Jurassic period, though no fossil evidence of metatherians themselves are known from this time. The oldest metatherian fossils are found in present-day China. Metatherians are thought to have then spread westward into modern North America, where the earliest true marsupials are found.

In South America, the opossums evolved and developed a strong presence, and during the Palaeogene also saw the evolution of shrew opossums (Paucituberculata) alongside non-marsupial metatherian predators such as the borhyaenids and the sabre-toothed Thylacosmilus.

Marsupials reached Australia via Antarctica at least 50-65 million years ago suggesting a single dispersion event of just one species, most likely a relative to South America's monito del monte (a microbiothere, the only New World australidelphian). In Australia, they radiated into the wide variety seen today. Modern marsupials appear to have reached the islands of Borneo and Sulawesi relatively recently via Australia.

A 2010 analysis of retro-transposon insertion sites in the nuclear DNA of a variety of marsupials has confirmed all living marsupials have South American ancestors. The branching sequence of marsupial orders indicated by the study puts Didelphimorphia in the most basal position, followed by Paucituberculata, then Microbiotheria, and ending with the radiation of Australian marsupials. This indicates that Australidelphia arose in South America, and reached Australia after Microbiotheria had split off.