A relatively new ergogenic aid called creatine is surfacing among collegiate and elite level athletes. Creatine is advertised as an energy enhancing molecule. Yet the athletes who supplement their diets with it are likely to possess limited information regarding the reasons for supplementation. After a brief review of creatine's physiological function, supplementation with creatine monohydrate will be discussed, focusing on its specific muscular and exercise effects. Finally, recommendations will be made for the athlete. The Physiology of Creatine and Phosphocreatine. Creatine, first discovered in 1832, is a naturally-occurring molecule produced from the amino acids glycine, arginine, and methionine in the liver, kidneys, and pancreas. Blood transport shuttles the creatine to its useful site in the skeletal muscle fibers, where 95 percent of the total creatine in the body is located. Once inside the body, creatine is stored, predominantly in skeletal muscles. The body maintains a storage pool of approximately 120g, two thirds of which is stored in the phosphocreatine form and one third in the free creatine form (1). As might be expected, the body does not store all of the ingested creatine; rather the excess is converted to creatinine and lost in urine. In fact, the average male has a turnover rate of approximately 2g/day, which is counterbalanced by an intake close to 1g/day of creatine from various animal products present in the diet. Creatine functions in two ways, as a storage site for high energy phosphates to buffer shifts in the adenosine triphosophate (ATP) dephosphorylation reaction and as a phosphocreatine shuttle (2). The removal of this phosphate from an ATP molecule produces the energy required for muscular contraction. Even more important is the reverse reaction in which phosphocreatine rejoins a phosphate with adenosine diphosphate (ADP), thereby yielding another ATP molecule available for energy production. Creatine stores the high energy phosphate and helps regulate the equilibrium of the reaction by phosphorylating an ADP when it is thermodynamically favorable. Greater stores of phosphocreatine allow for phosphorylation of ADP at an increased rate and thus improved physical performance.