There are 23 described species of possum in Australia and at least as many species in New Guinea. A related species is sufficiently different to warrant its own superfamily -- the honey-possum of south-western Australia.
The possums are divided into three major families: the ringtail possums and gliders (Family Petauridae); the brushtail possums and cuscuses (Family Phalangeridae) and the pygmy-possums and feathertail glider (Family Burramyidae). The fine details of the relationships between the possums is still unclear and no doubt new findings will result in changes to their classification.
The Australian possums were so named because of their supposed similarity to the opossums of South America. In fact, the two groups are not closely related apart from both being aboreal (tree-dwelling) marsupials. Indeed, some old-timers still refer to these animals as opossums and Opossum Bay, an outer suburb of Hobart, takes its name from the misnomer.
Tasmania is home to five species of possum. They are: