Osteoporosis is defined as a decrease in bone mass accompanied by structural changes, leading to an increase in fracture propensity. Early diagnosis of osteoporosis, fracture risk prediction, and assessment of efficacy of therapy therefore are of great interest. A number of noninvasive techniques are available for measuring bone mass at multiple sites of the skeleton. This article reviews basic methodology and developments in radiology such as x-ray and photon absorptiometry and quantitative computed tomography, which are routinely used in clinical practice. Recent techniques for assessment of bone mineral density and structure, such as ultrasound measurements, and their possible clinical applications also are discussed.