For most of the 20th century, scientists have believed that protein needs are not altered by physical exercise. In contrast, athletes are typically convinced that additional dietary protein can significantly enhance exercise performance. Until recently, the opinion of the athletes has been largely unsubstantiated in the scientific literature. However, since the 1970s, an increasing number of studies have appeared that indicate dietary protein needs are elevated in individuals who are regularly physically active. Together, these data suggest that the RDA for those who engage in regular endurance exercise should be about 1.2-1.4 g protein/kg body mass/d (150-175% of the current RDA) and 1.7-1.8 g protein/kg body mass/d (212-225% of the current RDA) for strength exercisers. Fortunately, the typical North American diet contains protein near these quantities, so most individuals who decide to begin an exercise program will obtain sufficient protein as long as their diet is mixed and they are careful to consume adequate energy. Populations at greatest risk for consuming insufficient protein include any group that restricts energy intake (those on diets) or high quality protein sources (vegetarians) as well as any group that has a requirement higher than normal due to another existing condition (growing individuals). Future studies should focus on these groups. Moreover, few exercise performance measures have been made, so any negative effect of insufficient dietary protein on athletic success needs to be determined. Supplementation of several individual amino acids may be beneficial for physically active individuals, but considerable potential risk is also present. Intake of large quantities of individual amino acids is not recommended until much more information is available.