The northern flying squirrel is one of three species of the genus Glaucomys, the only flying squirrels found in North America. They are found in coniferous and mixed coniferous forests across much of Canada, from Alaska to Nova Scotia, and south to the mountains of North Carolina and west to Utah in the United States. They are light brown with pale underparts and grow to a length of 25 to 37 cm. They are proficient gliders but uncoordinated walkers on the ground. They feed on a variety of plant material as well as tree sap, fungi, insects, carrion, bird eggs and nestlings. They mostly breed once a year in a cavity lined with lichen or other soft material. Except when they have young, they change nests frequently, and in winter a number of individuals may huddle together in a shared nest. Unlike most members of their family, flying squirrels are strictly nocturnal.

The three species of New World flying squirrels, genus Glaucomys, are the only species of flying squirrel found in North America. They are distributed from Alaska to Honduras. They are similar in many ways to the Eurasian flying squirrels in the genus Pteromys. Two species of New World flying squirrels can be easily distinguished on the basis of size and ventral pelage. Northern flying squirrels, Glaucomys sabrinus are larger and have belly hair that is dark at the base and white at the tip. Southern flying squirrels, Glaucomys volans, are smaller and have belly hairs that are completely white. Humboldt's flying squirrel is more difficult to distinguish from the northern flying squirrel where their ranges overlap. In fact, they were once considered conspecific. Humboldt's flying squirrel is considered a cryptic species. They are generally smaller and darker than northern flying squirrels.