This article describes the relationship of sarcopenia and dynapenia with three important outcomes in aging research: functional status, falls, and mortality. The data from epidemiologic studies conducted in large samples of older men and women suggest that muscle functioning, as indicated by muscle strength or muscle power, has a strong impact on functional status, falls, and mortality. Furthermore, there is evidence that the relationship between poor muscle strength and these three different outcomes is not influenced by muscle size. For the prevention of functional decline, falls, and early mortality in older men and women a major focus on maintaining or increasing muscle strength instead of muscle size seems warranted.