Water has thermal properties, since it helps to maintain body temperature constant, especially during prolonged physical exercise in a hot and humid environment. The main cation lost through sweat is sodium. The authors review the causes of exercise-induced fluid loss (dehydration, hypohydration and hyponatremia) as well as its consequences (firstly, a reduction in sporting performance and secondly, muscle and tendon functional impairment). The distinct modalities and recommendations for adequate hydration before and during exercise, as well as for rehydration after exercise, are reviewed. The advantages of postexercise rehydration with moderately high sodium-containing drinks (0.15 g/100 ml) in comparison with low sodium-containing drinks (0.07 g/100 ml), together with intake of 150% of the weight lost through dehydration, are discussed.