Skeletal muscle (SM) plays a central role in many biological functions, such as movement and metabolism, and alterations in SM mass and composition can have a marked influence on health and disease. The purpose of this review was to examine and characterize age-related changes in SM mass and composition and to link these changes to corresponding changes in metabolism and disease. It can be concluded from several studies that SM mass decreases with advancing age in both men and women. The age-related reduction in SM, also known as sarcopenia, is related to a corresponding reduction in SM strength, and if left untreated, sarcopenia may lead to functional impairment and physical disability. An emerging body of evidence also suggests that age-related changes in the composition of SM, particularly the accumulation of lipids within SM fibers, may lead to metabolic disorders such as insulin resistance.