Dual-energy projection methods have been used over the past decade for non-invasive measurement of bone. Dual-photon absorptiometry (DPA) using a 153-Gd radionuclide source is used clinically in about 1000 centers to measure regional bone mineral density (BMD), particularly of the spine and proximal femur (Barden and Mazess, 1989). The sane method has been applied to measurement of the total body bone mineral content (BMC) and BMD (Peppler and Mazess, 1981; Mazess et al., 1984b; Nuti et al., 1987; Gotfredsen et al., 1984a, 1984b, 1986). Total body scans using DPA require about 60–80 minutes; the radiation dose is low (1 mrem). Total body BMC correlates highly with actual skeletal mass and with total body calcium by neutron activation analysis in vivo (Mazess et al., 1981; Heymsfield et al., 1989) because calcium is a constant fraction (about 38%) of the mineral component, or calcium hydroxyapatite. The typical precision error (1 SD) for total body BMD is under 1% (Nijs et al., 1988; Mazess et al., 1988; Gallagher et al., 1987). In the past year, new instrumentation was developed in which the two monoenergetic peaks formerly supplied by the radionuclide source are provided by an x-ray generator and K- edge filter (Mazess et al., 1989; Sartoris and Resnick, 1989). This new methodology is called dual-energy x-ray absorptiometry (DEXA). DEXA provides total body scans in a fraction of the time required for DPA (10 vs 70 minutes), yet the precision, spatial resolution, and radiation dose (<0.02 mrem) are better.