Aging skeletal muscle exhibits decreases in muscle mass and force and changes in contractile properties. The effects of aging on the physiological characteristics of skeletal muscle are fiber type specific. This review describes the aging process in skeletal muscle; specifically, the effects of aging on the biochemical, morphological, and physiological characteristics of type I (slow-twitch) fibers and type II (fast-twitch) fibers. The effects of training on specific fiber types are also reviewed. The age-related decrease in maximum isometric force may be due, in part, to a decline in muscle mass. Decreases in muscle mass appear to occur in weight-bearing muscles and are most marked in those with a high proportion of type II fibers. The age- related fiber atrophy contributes to the decline in muscle mass. The decline in fiber size is prominent in type II fibers, whereas type I fibers are less affected. The age-related prolongation in isometric twitch properties may be due, in part, to alteration in the capacity of the sarcoplasmic reticulum for calcium release and recapture. Resistance and endurance training appear to attenuate the age-related alterations in skeletal muscle properties if the stimulus is of a sufficient intensity and duration.