Ultrasound has been investigated as a tool for characterizing the biomechanical competence of bone. The rationale for using ultrasound rests on two points. First, its interaction with tissues can be used to measure their density, velocity, and structure, and thus to characterize the elasticity and to infer the strength of bone. Second, ultrasound may be used to characterize tissue properties over a wide range of spatial dimensions and organizational levels, ranging from its constituents (e.g., trabeculae for cancellous bone) to the entire organ. Different ultrasound techniques can be used to investigate diverse bone properties, but two techniques have emerged as having the potential for providing useful information on problems of current biomedical interest. These measure two parameters, density and velocity, which correlate with the elastic and ultimate properties of bone. In particular, the elasticity E is formally related to the product of density and velocity squared, E = rho v2. Moreover, it has been shown by mechanical testing that there is a single linear correlation between elasticity and strength at all orientations, both in cortical and in cancellous bone, materials with a strong intrinsic anisotropy. At the tissue level, it may therefore be expected that the ultrasound parameters will prove to be useful predictors of bone strength and of its dependence on orientation. In vitro ultrasound studies have shown that these properties can be measured specifically and quantitatively, and that they vary under different physiological conditions.