Important advances have been made in the understanding of the regulation of protein metabolism, which are of relevance to those interested in maximizing muscle performance in sport and aging. The use of 24 h amino acid infusion studies, the development of methods to measure skeletal muscle fractional breakdown rate and the fractional synthetic rate of specific proteins have and will continue to enhance our understanding of protein metabolism in exercise. Recent studies have demonstrated potentially beneficial effects upon protein metabolism by altering the composition and timing of nutrient delivery. Exercise and nutritional interventions may positively influence the negative effects of aging upon protein metabolism.