Rather than the age-old debate regarding overall protein and amino acid needs of athletes, this paper focuses on the importance of timing and type of protein and amino acid ingestion relative to both muscle growth and exercise performance. Evidence discussed comes from definitive measurement techniques including net protein balance determinations (for acute studies) or quantification of muscle size or strength (for chronic studies) First, recent data indicate that consuming a small meal of mixed macronutrient composition(or perhaps even a very small quantity of a few indispensable amino acids) immediately before or following strength exercise bouts can alter significantly net protein balance, resulting in greater gains in both muscle mass and strength than observed with training alone. With aerobic exercise, some evidence suggests immediate postexercise (but perhaps not pre-exercise) supplementation is also beneficial. Second, protein type may also be important owing to variable speeds of absorption and availability, differences in amino acid and peptide profiles, unique hormonal response, or positive effects on antioxidant defense. In addition to athletes, many others who desire to regain, maintain, or enhance muscle mass or function, including those with muscle-wasting diseases, astronauts, and all of us as we age, need to ensure that nutrient availability is sufficient during the apparently critical anabolic window of time associated with exercise training sessions. Future studies are needed to fine tune these recommendations.