This review provides a critical evaluation of the metabolic rationale for the use of individual amino acids as nutritional ergogenic (work-generating) aids in athletes. The conclusion is that in contrast to the claims made on sport nutrition products, branched-chain amino acids do not improve endurance performance, that the evidence that glutamine supplements may improve immune function is rather weak, and that the available commercial supplements contain too little arginine to increase growth hormone levels. No studies have been performed to investigate the claim that tyrosine supplements can improve explosive exercise.