The strength of bone depends on bone structure, both in terms of global geometry of the bone and the microscopic pattern of the trabecular network. Modifications in the trabecular organization, often related to the aging process, include altered anisotropy, perforations, loss of connectivity and microfractures. The microscopic architecture of trabecular bone can be studied on histomorphometric slices, the use of stereologic techniques on these slices may better reflect the changes occurring in 3 dimensions. High-resolution tomodensitometry and 3-D magnetic resonance imaging are under further evaluation. An evaluation of the fractal geometry of the trabecular network, performed on the vertebrae, calcaneus, or ulna on standard radiographs has given promising results. Bone strength is determined by resistance tests on bone tissue samples. The relationship between the applied force and bone deformation is used to determine a stress-strain curve and resistance parameters. The mechanical behavior of a given bone can be modelized by a Finite Element Analysis, the computerized model being able to predict the reaction of this bone. Ultra-sound analysis does not directly measure bone density, nor bone architecture, but would reflect both these properties. Velocity and attenuation of ultra-sounds, measured at the calcaneus or patella, are useful in distinguishing between controls and osteoporosis cases, and may have predictive value.