This chapter provides an overview of hydration of the athlete's body. Specific functions that water serves in the body include providing the medium for metabolic reactions, serving as a substrate or a reactant for specific reactions, transporting nutrients and waste products to specific locations, contributing in several ways to thermoregulation, and contributing to lubrication. The volume is regulated through a complex system of exchange of fluids, solutes, and ions within the compartments in the body and also influenced by the environment. The external environment influences body water balance through air temperature, humidity, atmospheric pressure, microgravity, and hyperbaric conditions. Spaceflight promotes a negative body water balance as a result of several different physiological stressors during the take-off: hyper-gravity g-forces, the microgravity environment itself, and the hydration status of the individual along with psychological stress. When the human body is exposed to higher than normal ambient temperatures, skin vessels vasodilate to increase blood flow and facilitate heat loss via conduction and convection.