Sarcopenia is the loss of muscle mass and strength that occurs with normal aging. Because sarcopenia is not the result of a disease, it is seen in all aged adults. Sarcopenia markedly increases the risk of disability and loss of functional capacity in the elderly. The mechanisms underlying sarcopenia are complex and are reviewed here. It is not clear at this time which factors are most important in determining the severity or rate of development of sarcopenia. While progressive resistance training clearly can reverse and prevent sarcopenia, little is known about the mechanisms by which aged muscle adapts to training, or whether these adaptations reflect reversal of direct pathophysiological processes or compensation by activation of separate pathways from those leading to the deterioration in the first place. As populations in developed countries continue to age, diagnosing, treating, and preventing sarcopenia will be progressively more important to the health and well-being of modern societies. Key words: aging, muscle, cytokines