Osteoporosis is a skeletal disorder that predisposes individuals to increased risk of fracture. However, most osteoporotic fractures occur in women who do not meet criteria for osteoporosis. Hence, bone density, by itself, is a relatively poor predictor of fracture. Age and age-related factors are now recognized as increasingly important in determining fracture risk. Osteoporotic fractures are associated with increased disability and mortality, suggesting that osteoporosis may be a clinical manifestation of an underlying disease process affecting multiple systems. The systems affected, the musculoskeletal system and the central nervous system, are shared in many respects with the frailty syndrome. Vitamin D deficiency is a major contributor to the frailty syndrome, osteoporosis, and osteoporotic fractures. Its effects are mediated by the development of cerebrovascular disease, postural instability, muscle weakness, and bone fragility. Thus, osteoporotic fractures result from both a bone and brain disease.