This article summarizes a variety of noninvasive techniques for measuring bone density in the clinical setting. Although dual-energy x-ray absorptiometry and quantitative computed tomography are currently the most widely used methods, older techniques such as radiogrammetry and single-photon absorptiometry continue to have research applications, and experimental approaches such as compton scattering and proton activation analysis may be important in the future. The evaluation of bone mass measurement underscores continuing progress in our understanding of bone metabolism and biomechanics, and has provided valuable insight into improvements in the therapy of osteoporosis and other forms of metabolic bone disease.