Bone is hard tissue that is in a constant state of flux, being built up by bone-forming cells called osteoblasts while also being broken down or resorbed by cells known as osteoclasts. During childhood and adolescence, bone formation is dominant; bone length and girth increase with age, ending at early adulthood when peak bone mass is attained. Males generally exhibit a longer growth period, resulting in bones of greater size and overall strength. In males after the age of 20, bone resorbtion becomes predominant, and bone mineral content declines about 4% per decade. Females tend to maintain peak mineral content until menopause, after which time it declines about 15% per decade. Osteoporosis is a disease characterized by low bone mass and structural deterioration of bone tissue, leading to bone fragility and an increased susceptibility to fractures, especially of the hip, spine, and wrist. Osteoporosis occurs primarily as a result of normal ageing, but can arise as a result of impaired development of peak bone mass (e.g. due to delayed puberty or undernutrition) or excessive bone loss during adulthood (e.g. due to estrogen deficiency in women, undernutrition, or corticosteroid use). Osteoporosis-induced fractures cause a great burden to society. Hip fractures are the most serious, as they nearly always result in hospitalization, are fatal about 20% of the time, and produce permanent disability about half the time. Fracture rates increase rapidly with age and the lifetime risk of fracture in 50 year-old women is about 40%, similar to that for coronary heart disease. In 1990, there were 1.7 million hip fractures alone worldwide; with changes in population demographics, this figure is expected to rise to 6 million by 2050. To help describe the nature and consequences of osteoporosis, as well as strategies for its prevention and management, a WHO Scientific Group meeting of international experts was held in Geneva, which resulted in this technical report. This monograph describes in detail normal bone development and the causes and risk factors for developing osteoporosis. The burden of osteoporosis is characterized in terms of mortality, morbidity, and economic costs. Methods for its prevention and treatment are discussed in detail for both pharmacological and non-pharmacological approaches. For each approach, the strength of the scientific evidence is presented. The report also provides cost-analysis information for potential interventions, and discusses important aspects of developing national policies to deal with osteoporosis. Recommendations are made to the general population, care providers, health administrators, and researchers. Lastly, national organizations and support groups are listed by country.