The athlete with exertional heat cramps may be dehydrated, but the overriding issue that needs to be urgently addressed is the sweat-induced exchangeable sodium deficit. However, for myriad athletes and athletic scenarios, it is impractical to avoid accumulating and sometimes measurable total body water and exchangeable sodium deficits during extended or repeated same-day bouts of training or competition. Considering these sweat loss rates, it is easy to appreciate how some individuals can readily incur a sizable total body water deficit during training and competition. While the primary determinant of thermal strain is the combined effect of intensity and duration of physical activity and environmental heat stress, sufficient rehydration and sodium intake are essential in minimizing total body water and exchangeable sodium deficits and thus mitigating thermal strain and exertional heat illness risk. In addition to the potential effect on performance, these deficits can significantly increase thermal strain and exertional heat illness risk.