Osteoporosis is a disorder characterized by low bone density and impaired bone strength which is an important risk factor for fracture in older adults. The diagnosis of osteoporosis in postmenopausal women is now based on bone density testing by dual energy x-ray absorptiometry but not other methodologies. However, a specific but arbitrary diagnostic threshold must be distinguished from a strategy to assess fracture risk. In untreated postmenopausal women and older men, bone density is an important, but not the only, determinant for fracture risk. Combining bone density measurements with other independent and validated risk factors for fracture provides a much more accurate assessment of an individual patient's risk for fracture than does bone density alone. The most important of these other risk factors are age and prior fracture history. Clinical guidelines will move away from recommending treatment at specific T scores toward intervention thresholds based on absolute fracture risk. By basing who to treat on fracture probability, therapy can be targeted to those patients who would receive the greatest benefit.