Osteoporosis is the reduction of expected bone mass. This results in structural failure with an increased risk of fracture and it is the most common bone disorder encountered. Bone mass declines with age, and in some people will fall below the threshold for easy fracture. This loss is accelerated in the postmenopausal period. Trauma and the internal trabecular structure of bone are additional determinants of risk of fracture. Effective management of osteoporosis depends on identifying and treating those at risk before reaching the critical bone mass and presenting with skeletal failure. There are limitations to methods available for assessing bone loss, and the final arbiter of any treatment is prevention of fracture. Primary prevention involves maximising peak adult bone mass and reducing the rate of bone loss. This may be attained by exercise, adequate dietary calcium, and the identification and treatment of risk factors such as postmenopausal hormone replacement. Once skeletal failure has occurred, long term treatment is required to have a clinically significant effect. Increasing bone mass cannot be assumed to reduce the risk of fracture, and such a reduction has not been directly demonstrated for several agents. Calcium supplements, hormone replacement therapy and fluoride are probably effective in reducing fracture rate, particularly when used in combination, whereas the efficacy of anabolic steroids, calcitonin and diphosphonates is yet to be established. Vitamin D is only of use in coexistent osteomalacia. The limitation of significantly strengthening the skeleton during the life expectancy of the elderly must be realised.