Active persons ingest protein supplements primarily to promote muscle strength, function, and possibly size. Currently, it is not possible to form a consensus position regarding the benefit of protein or amino acid supplements in exercise training. Determination of whether supplements are beneficial has been hampered by the failure to select appropriate endpoints for evaluation of a positive effect. Furthermore, studies focused at a more basic level have failed to agree on the response of protein metabolism to exercise. An additional complication of dietary studies that is not often taken into account is amount of energy intake. Because of these and other complications, studies at the whole body level have not yielded a clear picture of the need for, or response to, dietary protein or amino acid supplements. Consequently, it is necessary to examine this issue at the tissue level. In untrained subjects, both muscle protein breakdown and synthesis are increased in response to exercise. Amino acid intake further stimulates muscle protein synthesis after exercise as a consequence of stimulating amino acid transport into the intramuscular compartment. The stimulatory effect of amino acids after exercise is greater than the effect of amino acids on muscle protein synthesis when given at rest. These data suggest that not only may the exact composition and amount of an amino acid supplement be important, but the timing of ingestion of the supplement in relation to the exercise must be considered in designing future studies to evaluate the efficacy of amino acid supplements.