Abstract: Computed tomography (CT) can yield quantitative imaging data from detailed maps of linear attenuation coefficients within tissue. The attenuation characteristics of skeletal muscle and adipose tissue can be quantified in vivo to provide information about the composition of skeletal muscle and the distribution of adipose tissue within muscle. Several studies have taken advantage of this utility to quantify skeletal muscle composition and fatty infiltration of muscle, in particular to quantify the attenuation characteristics of muscle as a marker of its lipid content. In this manner we found that the mean muscle attenuation of skeletal muscle reflects an increase in its fat content in obesity, and that this regional body composition parameter is strongly related to insulin-resistant glucose metabolism. In addition, muscle composition and adipose tissue distribution within muscle may be altered with clinical weight-loss interventions. CT may also provide important information about the changes in muscle mass and composition with aging and disease, which may, in turn, affect the muscle's function. In summary, CT can provide important quantitative data on the composition of muscle, and the distribution of adipose tissue within it, and this may be important in examining the relationships among skeletal muscle metabolism, lipid accumulation within muscle, and muscle function.