Increasing the relative amount of protein in the diet of athletes has been suggested to optimise anabolic processes and improve both physiological responses to training and performance. While energy balance studies generally support the concept that athletes may require additional protein in their diets in comparison with the Recommended Dietary Allowances (RDA), most sport nutritionists contend that as long as athletes maintain energy balance and ingest 15% of their total caloric intake in the form of protein, additional supplementation of protein is not necessary. Recently, amino acids have become a popular nutritional supplement marketed to athletes. In strength athletes, amino acid supplementation has been proposed to increase the availability of essential amino acids, enhance anabolic processes promoting tissue accretion, and accelerate the rate of recovery during training. In endurance athletes, amino acid supplementation has been proposed to improve physiological and psychological responses during endurance exercise and training. There appears to be little scientific evidence to support the hypothesis that amino acid supplementation may enhance the physiological responses to strength training when athletes consume dietary protein within the recommended guidelines. Results of the effects of amino acid supplementation on the physiological and psychological responses to endurance exercise are preliminary. However, the findings suggest that amino acid supplementation with carbohydrate before, during and after exercise may alter the ratio of free tryptophan to branch-chained amino acids. Further research is required before definitive conclusions can be drawn regarding the proposed ergogenic value of amino acid supplementation.