Noninvasive measurements of bone mineral density allow the assessment of skeletal integrity, both centrally and peripherally, with high precision and accuracy and with relatively low radiation dose. When estimating skeletal status, it may be important to measure bone mineral density at more than one site to assess differential skeletal responses related to disease or therapy and to assess differential fracture risk. Due to technical differences between the various methods of bone mineral measurement, the quantitative results are typically expressed with differing calibration standards, such that direct comparisons must be carefully made. SPA measurements have been shown in several prospective studies to aid in the assessment of osteoporotic fracture risk. Limited data to date have shown spinal DPA to be at least comparable to peripheral SPA for fracture risk assessment, and current research with DXA indicates promising results for the X-ray-based bone densitometers. DXA has seen rapid growth in recent years, with current scanners able to measure the spine, hip, forearm and total body bone mineral density with a speed and precision previously unattainable with the isotope-based DPA systems. Longitudinal studies have shown QCT to be highly sensitive for detecting early and rapid bone loss and cross-sectional studies have shown QCT's capacity for separating normal and osteoporotic patient populations. though prospective studies are needed to confirm the latter result. QCT has the disadvantage of higher cost and radiation dose compared with the other methods currently in use, but it is the only noninvasive modality able preferentially to measure trabecular, cortical, or integral bone density at any skeletal site. All of the techniques in current clinical use, specifically SPA, DPA, DXA and QCT, represent major advances for the noninvasive measurement of bone mineral density at radiation doses significantly less than those due to yearly exposure from normal background radiation.