To assess the age-related loss of muscle mass and to determine the mechanisms behind this aging atrophy, the muscle structure and fiber type composition have been estimated, using invasive and noninvasive techniques. Limb muscles from older men and women are 25–35% smaller and have significantly more fat and connective tissue than limb muscles from younger individuals. Comparisons of muscle biopsies from younger and older individuals reveal that type 2 (fast-twitch) fibers are smaller in the old, while the size of type 1 (slow-twitch) fibers is much less affected. Studies of whole muscle cross sections also show a significantly smaller number of muscle fibers, a significantly lower relative type 2 fiber area, and a significant increase in fiber type grouping with increasing age. These results indicate a gradual decrease in sizelvolume with advancing age, accompanied by a replacement by fat and connective tissue. This aging atrophy seems to be due to a reduction in both number and size of muscle fibers, mainly of type 2, and is to some extent caused by a slowly progressive neurogenic process.