The squirrel family (Sciuridae) is a diverse group of about 50 genera of rodents, including the "true" or tree squirrels, as well as flying squirrels, ground squirrels, chipmunks, marmots, woodchuck, and prairie dogs. Members of the squirrel family occur in North and South America, Africa, Eurasia, and Southeast Asia, but not in Madagascar, New Guinea, Australia, or New Zealand.

The squirrel family encompasses species that are exclusively arboreal, living in tropical, temperate, or boreal forests. It also includes species that are exclusively terrestrial, living in burrows in the ground in alpine or arctic tundra, semiarid desert, prairie, or forest edges. Most squirrels are diurnal, but a few, such as the flying squirrels, are nocturnal. Most squirrels are largely herbivorous, eating a wide variety of plant tissues. Some species, however, supplement their diet with insects, bird eggs, and nestlings.

The following sections describe most of the major groups in the squirrel family, with an emphasis on species occurring in North America.

Tree squirrels

There are about 55 species of tree squirrels in the genus Sciurus, that occur in Asia, Europe, North America, and South America. As their name suggests, tree squirrels are highly arboreal animals, living in forests of all types, from the limits of trees in the north, to the tropics.

Tree squirrels have a long, bushy tail, used as a rudder when they are airborne while leaping from branch to branch and as a comfy wrap-around when the animal is sleeping. Tree squirrels forage during the day. They eat a wide range of plant tissues, but are partial to the flowers, nuts, and fruits of trees, sometimes foraging on the ground to obtain these after they have fallen. They may also feed on insects, bird eggs, and nestlings.

Tree squirrels utter a loud barking chatter when alarmed, often accompanied by an agitated fluttering of their tail. Their color varies from black through red, brown, and grey, often with whitish underparts. Tree squirrels do not hibernate, but they may sleep deeply in their arboreal nests for several days running during inclement winter weather.

The eastern grey squirrel (Sciurus carolinensis) is a widespread species in eastern North America. Although grey is the most common color of the pelage of this species, black-colored animals also occur, and these can be dominant in many eastern populations. The grey squirrel is found mostly in temperate angiosperm and mixed conifer-hardwood forests, but it has also adapted well to habitats available in the urban forests of older, more-mature neighborhoods.

The western grey squirrel (S. griseus) occurs in oak and oak-pine forests of the western states. The eastern fox squirrel (S. niger) is a resident of hardwood forests of the eastern United States. This relatively large grey or rusty-yellowish colored species is commonly hunted as a small-game animal. The tassel-eared or Kaibab squirrel (S. alberti) occurs in pine forests of upland plateaus of the central southwestern States, and has long, distinctive, tufts of hair on the tops of its ears.