The latest micro-computed tomography (micro-CT) can provide ultrahigh-resolution images with resolution of less than 10 microm. Geometric three-dimensional (3-D) parameters, including the orientation, shape, and connectivity of trabeculae, are particularly helpful in understanding complex 3-D structure. Micro-CT is helpful for studying bone changes in various pathophysiological states and assessing changes in microarchitecture after treatment with antiosteoporotic agents. Trabecular microarchitecture is strongly related to bone strength, and 3-D micro-CT data can be used to assess bone biomechanical properties with the help of finite-element analysis. High photon flux from synchrotron X-ray sources reveals precise bone surface structure, and the monochromaticity of the beam is suitable for performing accurate density measurements. The preliminary results of an in vivo study of microarchitecture are also shown. Human vertebral microstructure can be revealed using multidetector row CT at a resolution of 200 x 200 x 300 microm. Analysis using high-resolution CT microstructure was found to be more useful in identifying subjects at high risk of fracture than clinical bone density measurements using dual X-ray absorptiometry.