Fluid balance in the body is the balance between water input (from foods, beverages , and a small amount generated by metabolism) a nd water output (urine, insensible losses, sweat, and fecal loss). During conditions of physiological stress (ie, exercise), the ability to maintain optimal fluid balance (ie, euhydration) has significant implications with regard to body temperature regulation. The body's response to increased body temperature during exercise is to increase skin blood flow and sweating. The ability of these thermoregulatory responses to optimally attenuate increases in body temperature is affected by hydration status. For example, a deficit in fluid balance (ie, dehydration) results in a higher core body temperature than if euhydration is maintained. A higher core body temperature can negatively affect athletic performance, mood state, and cognitive function and can increase one's risk for heat illness. This risk is further exacerbated when extreme environmental conditions place additional stress on thermoregulatory function (ie, hot/dry or hot/ humid conditions; temperature ≥ 90°F, 32°C). This chapter will focus on and provide the scientific evidence for the relationship between hydration status and core body temperature during exercise. C o p y r i g h t e d m a t e r i a l. N o t f o r d i s t r i b u t i o n .