Sarcopenia is the loss of muscle mass and strength with age. Sarcopenia is a part of normal aging, and occurs even in master athletes, although it is clearly accelerated by physical inactivity. Sarcopenia contributes to disability, reduced ability to cope with the stress of a major illness, and to mortality in the elderly. The etiology of sarcopenia is unclear, but several important factors have been identified. These include loss of alpha motor neurons, decline in muscle cell contractility, and several potential humoral factors, such as androgen and estrogen withdrawal and increase in production of catabolic cytokines. Treatment of sarcopenia with progressive resistance training is safe and effective, but dissemination of this technique to the general population has yet to occur. As the number of elderly persons increases exponentially in the new century, a public health approach to prevention and treatment of sarcopenia, based on increasing physical activity at all ages, will be crucial to avoiding an epidemic of disability in the future.