Fluid-electrolyte deficiencies may accumulate over several days in athletes whose diets contain nutrients (eg, water, sodium, potassium) in quantities that are smaller than the losses that occur in sweat and urine. Individualized assessment and personalized dietary recommendations are necessary, especially in cases of water or salt depletion, heat exhaustion, and heat cramps. Unfortunately, no composite source of these techniques exists. The purpose of this article is to describe methods that validly assess urine and sweat volumes, electrolyte concentrations, dietary fluid and electrolyte intake, and 24-hour water and electrolyte balance for athletes who train and compete in hot environments. Reference (ie, expected) values, sample data sheets, and a case report are presented. These techniques allow motivated professionals to validly characterize the fluid-electrolyte status of athletes and thereby suggest modifications of diet and training habits. Without these techniques, professionals must resort to inaccurate or indirect estimates of whole-body fluid and electrolyte balance.