Following mating and fertilization, the gestation period lasts for about 38 to 44 days. The young are hairless and helpless at birth and are nursed by the mother. They grow hair after about two weeks, and the eyes open after four to five weeks. They leave the nest to be on their own six to eight weeks after birth and are ready to breed the following year. Some tree squirrels have two litters a year that average about three young each.
Ground squirrels have a single litter a year that averages from four to nine young.
The New World flying squirrels normally have two to four young twice a year, following a gestation period of five to six weeks. The young open their eyes before they are four weeks old and leave the nest about two months after birth. Little is known about the reproductive habits of most other species of squirrels, particularly the tropical forms that are less common and more difficult to study. The giant flying squirrel of Malaysia usually gives birth to only one or two young in a litter. Some of the tropical species of squirrels are known to bear young several times during a year.