Single and dual photon absorptiometry are now available to aid the physician in diagnosis and management of generalized bone diseases. Both methods are reviewed in detail. They allow an accurate and reproducible assessment of bone mineral at predominantly cortical (appendicular skeleton) and trabecular (axial skeleton) bone sites. Newer findings show that cortical and trabecular bone behave differently and that the effect of endocrine dysfunction on bone appears to be both disease and site specific. These qualitative and quantitative differences in cortical and trabecular bone loss require that an optimal sampling site has to be found for a given bone problem if maximum benefit is to be derived from these tests.