There is much discussion on what an athlete should be eating, but much less reference to drinking. Dehydration might decrease both aerobic and cognitive/mental performance. Overhydration might also impair physical ability, and pose a risk for exercise-associated hyponatremia. Athletes and coaches should anticipate possible fluid losses in prolonged events, and calculate appropriate drinking amounts to maintain euhydration. Body weight changes during similar events or training can be used to determine the amount of fluids to be ingested. Another important adjunct to sports nutrition is the wide range of sports dietary supplements. Such supplements are expected to enhance sport performance, by assisting either weight control, physical and mental performance (e.g. power, speed, endurance, agility), and recovery. Very few dietary supplements have been proven to possess clinically relevant effectiveness, and others are dangerous and illegal. It should be remembered that dietary supplements are not a substitute for a poor diet, that most are not produced or marketed by homogenous standards, that they are not under tight supervision, and that many are marketed without any proof of efficacy. Persons that are very active, whether professionally or recreationally, could benefit from consulting with sport dietitians in order to verify proper nutrition, hydration, and supplement use.