Ground squirrels are generally more robust than tree squirrels and often have short, sturdy forelimbs that are used for digging. Their tails, while fully furred, generally are not as bushy as those of tree squirrels. Like many other rodents, all squirrels have five functional toes on the hindfeet and four on the forefeet, with a well-developed claw on each digit. Squirrels occur throughout the world except for Australia, southern South America, and some desert regions. They inhabit a wide variety of habitats from tropical rain forest to arctic tundra and from the tops of trees to underground tunnels.
Most flying squirrels are nocturnal, other squirrels are nocturnal or diurnal.
The skulls of sciurids are short, with a short rostrum and arched profile. The postorbital processs are well developed, and the distance between the orbits is considerable. The jugals are long, and their anterior ends contact the frontals . The palate appears broad and relatively short, ending at about the same level as the molar row. Bullae are relatively large but not inflated.