A variety of pathophysiological mechanisms lead to bone loss. The decreased bone mass results in an increased risk of fractures, which are the sole clinical manifestation of osteoporosis. Thus, it is not surprising that osteoporosis should be seen as a complex, multifactorial chronic disease that may progress silently for decades until characteristic fractures result late in life. Because there are no symptoms until fractures occur, relatively few people are diagnosed in time for effective therapy to be administered. Consequently, a large number of individuals experience the pain, expense, disability and decreased quality of life caused by these age-related fractures. This important public health problem will worsen in the future as the population ages, unless suitable interventions can be devised. Preventing excessive bone loss is the only action that can be taken now to reduce fractures in the future, and the following chapters describe clinical strategies for identifying and treating those believed to be at greatest risk.