There is a rich scientific literature regarding hydration status and physical function that began in the late 1800s, although the relationship was likely apparent centuries before that. A decrease in body water from normal levels (often referred to as dehydration or hypohydration) provokes changes in cardiovascular, thermoregulatory, metabolic, and central nervous function that become increasingly greater as dehydration worsens. Similarly, performance impairment often reported with modest dehydration (e.g., -2% body mass) is also exacerbated by greater fluid loss. Dehydration during physical activity in the heat provokes greater performance decrements than similar activity in cooler conditions, a difference thought to be due, at least in part, to greater cardiovascular and thermoregulatory strain associated with heat exposure. There is little doubt that performance during prolonged, continuous exercise in the heat is impaired by levels of dehydration >or= -2% body mass, and there is some evidence that lower levels of dehydration can also impair performance even during relatively short-duration, intermittent exercise. Although additional research is needed to more fully understand low-level dehydration's effects on physical performance, one can generalize that when performance is at stake, it is better to be well-hydrated than dehydrated. This generalization holds true in the occupational, military, and sports settings.