The epidemiology of osteoporosis is reviewed in this article. Attempts were made to answer the following questions: How should osteoporosis be defined? How can risk factors and bone mineral density (BMD) measurements be applied to diagnose osteoporosis? How do the rates for osteoporotic fractures vary by country, sex, age and time? What are the costs for osteoporosis in terms of direct and indirect cost, morbidity and mortality? According to the WHO criteria, osteoporosis can be defined as a BMD of 2.5 standard deviations or more below the young normal mean. BMD measurements are predictive of fracture risks. Hip fracture is by far the most costly of osteoporotic fractures, and the rates are highest in Caucasians, intermediate in Asians and lowest in Blacks. Risk factors could be used to assist in the decision to measure BMD.