Osteoporosis is a widespread condition, often unrecognised in clinical practice, which may have devastating health consequences through its association with fragility fractures. Osteoporotic fractures represent an enormous public health burden. The total number of fractures, and hence the cost to society, will increase dramatically over the next 50 years as a result of demographic changes in the number of elderly people. This article reviews the latest advances in our knowledge of epidemiologic aspects of osteoporosis, including the epidemiology of fragility fracture, the determinants of fracture risk, and genetic, intrauterine, and environmental risk factors for osteoporosis. Novel relationships between breast cancer and bone mineral density will also be discussed.