Osteoporosis – an excessive decrease in bone mass – is more common in women than in men. It is a particularly common condition among elderly women in affluent countries. Osteoporosis is a risk factor for fractures, which occur most commonly at the wrist, spine and hip. Other important risk factors for fractures include those both related and unrelated to an excessive decrease in bone mass. Those related to an excessive decrease in bone mass include such causes as physical inactivity, smoking, low body weight, a history of fractures and the use of corticosteroids; those unrelated to bone mass loss include such causes as falls, high alcohol intake and visual impairment. Osteoporosis and the fractures associated with it are a major public health concern, because of related morbidity and disability, diminished quality of life, and mortality. The condition is responsible for about 1700 fractures a day (about 650 000 a year) in the European Union alone. Measures to prevent osteoporosis usually focus on a healthy lifestyle, which includes being physically active, no smoking, and taking adequate amounts of calcium and vitamin D. Pharmaceutical treatment in high-risk groups (such as people with an elevated risk of fracture) and measures to prevent falls are also proposed as important interventions for preventing fractures. Screening for osteoporosis, by measuring bone density or other measures, is suggested to identify and treat people at risk for fracture.