Osteoporotic fractures are a debilitating and a frequently fatal health problem for older adults. A growing body of evidence indicates that osteoporosis has its origin in early life and that the level of development of bone mass during childhood and adolescence strongly influences the risk for osteoporotic fractures. The development of osteoporosis results from an interaction between 1) bone mass accrual via growth, remodeling, and modeling during childhood and adolescence and 2) the maintenance of bone mass (primarily via remodeling) during adulthood. Peak bone mass which occurs at the conclusion of growth may be the most important factor for preventing osteoporosis since as much bone is accrued during the adolescent years as most individuals will lose during all of adult life. In this review, I examine the contribution of physical activity as an important behavioral determinant of children's bone development, particularly of peak bone mass. Since it is a behavior, physical activity is a potentially modifiable determinant of peak bone mass; therefore, understanding activity's impact on bone health is central to developing primary prevention strategies for osteoporosis.