Osteoporosis has long been considered a disease of the elderly; however, there is now a general agreement that predisposition begins in childhood and adolescence; thus, rational approaches to prevention of the disease should be started during childhood and adolescence. Indeed, by determining PBM, events occurring in the first two decades of life may determine in large part the subsequent risk of osteoporosis. Attention has thus been focused on the physiology of bone mass accumulation during growth, including the role of environmental factors such as dietary calcium and exercise. Because their patients are at this particular time of life, when PBM is being achieved, pediatricians are in a critical position to affect changes in the long-term risk of osteoporosis in their female and male patients.