Flying squirrels are long-limbed and slender and have large eyes; the long, bushy tail may be cylindrical or flattened. Their dense fur is soft and long and either silky or woolly in texture. A considerable range of body size exists among the 14 genera. Some giant flying squirrels of tropical India and southeastern Asia weigh 1 to 2.5 kg (2.2 to 5.5 pounds) and have a body length of about 30 to 60 cm (12 to 24 inches) and a tail 35 to 64 cm long. The smallest are the dwarf flying squirrels (Petaurillus) of northern Borneo and the Malay Peninsula; their bodies are just 7 to 9 cm long and their tails 6 to 10 cm. When seen in the tall trees of the tropical rainforest, the glides of these tiny rodents are easily mistaken for the flutter of large butterflies.

Unlike other squirrels, flying squirrels are nocturnal. They den in tree cavities, grottoes or rock crevices on cliffs, and cave ledges. Some also build globular nests high in trees where branches join the trunk. Nests are made of leaves, shredded bark, mosses, or lichens. Most species seldom leave the trees, but North American flying squirrels (Glaucomys) regularly descend to the ground to forage and bury nuts. Depending upon the species, diets can include seeds, fruit, leaves, flower buds, nuts, fungi, lichens, pollen, ferns, tree sap, insects, spiders, other invertebrates, small birds, eggs, snakes, and smaller mammals.

From high in a tree, the squirrel leaps into the air and extends its limbs to stretch the membranes, transforming the body into a gliding platform that is controlled by manipulating the membranes and tail. The animal sails downward to an adjacent tree. Just before the glide ends, it pulls upward, landing deftly on all four feet. When not in use, the membranes are pulled close to the body.

Flying squirrels constitute the subfamily Pteromyinae of the squirrel family (Sciuridae) within the order Rodentia (rodent). Some authorities regard flying squirrels as a family (Pteromyidae). A few species of anomalure are occasionally called scaly-tailed flying squirrels, but these rodents are classified in the family Anomaluridae. Some researchers have speculated that Pteromyinae is not a single group but members from two different clusters of tree squirrels or possibly nonsquirrel ancestors. Recent evidence derived from fossils and the anatomy of wrist and gliding membranes, however, indicates that all living flying squirrel species are closely related and likely evolved from a tree squirrel ancestor during the Oligocene Epoch (33.7 million to 23.8 million years ago).