Didelphimorphia is the order of common opossums of the Western Hemisphere. Opossums probably diverged from the basic South American marsupials in the late Cretaceous or early Paleocene. A sister group is Paucituberculata (shrew opossums). They are commonly also called possums, though that term is also applied to Australian fauna of the suborder Phalangeriformes. The Virginia Opossum is the original animal named opossum. The word comes from Algonquian wapathemwa.

Their unspecialized biology, flexible diet and reproductive strategy make them successful colonizers and survivors in unsettled times. Originally native to the eastern United States, the Virginia Opossum was intentionally introduced into the West during the Great Depression, probably as a source of food. Its range has been expanding steadily northwards, thanks in part to more plentiful, man-made sources of fresh water, increased shelter due to urban encroachment, and milder winters. Its range has extended into Ontario, Canada, and it has been found farther north than Toronto.

The Virginia Opossum (Didelphis virginiana) is the only marsupial found in North America north of the Rio Grande River. A solitary and nocturnal animal about the size of a domestic cat, it is a successful opportunist and is found throughout Central America and North America from coast to coast (introduced to California in 1910), and from Costa Rica to southern Canada; it seems to be still expanding its range northward. Its ancestors evolved in South America, but were enabled to invade North America in the Great American Interchange by the formation of the Isthmus of Panama about 3 million years ago. It is often seen near towns, rummaging through garbage cans, or lying by the side of the road, a victim of traffic.

The biological order Paucituberculata contains the six surviving species of shrew opossum: small, shrew-like marsupials which are confined to the Andes mountains of South America. It is thought that the order diverged from the ancestral marsupial line very early. As recently as 20 million years ago, there were at least seven genera in South America. Today, just three genera remain. They live in inaccessible forest and grassland regions of the High Andes. Insectivores were entirely absent from South America until the Great American Interchange three Ma ago, and are currently present only in the northwestern part of the continent. Shrew opossums have lost ground to the these and other placental invaders that fill the same ecological niches. Nevertheless, the ranges of shrew opossums and insectivores overlap broadly.

Shrew opossums (also known as rat opossums or caenolestids) are about the size of a small rat (9-14 cm long), with thin limbs, a long, pointed snout and a slender, hairy tail. They are largely carnivorous, being active hunters of insects, earthworms and small vertebrates. They have small eyes and poor sight, and hunt in the early evening and at night, using their hearing and long, sensitive whiskers to locate prey. They seem to spend much of their lives in underground burrows and on surface runways.

Largely because of their rugged, inaccessible habitat, they are very poorly known and have traditionally been considered rare. Recent studies suggest that they may be more common than had been thought.

Marsupials are an infraclass of mammals, characterized by a distinctive pouch (called the marsupium), in which females carry their young through early infancy.