This chapter presents the effect of aging on body water balance and thermoregulation and to attempt to develop recommendations on hydration and protection against these imbalances in master athletes. Water is a major component of our bodies. Adequate water content in the human body is of great importance in maintaining homeostasis and health from birth to old age and its volume is the resultant between its intake and output. Dehydration is a frequent geriatric problem, while access to water may be a matter of life or death for elderly people, even in the countries where droughts are only known from mass media. The maintenance of water–electrolyte homeostasis depends on the balance between the volume of water intake and water output. Water imbalance induces a number of responses, which lead to the adjustment of this disturbance. The water and electrolyte deficiencies are adjusted through a combination of changes in the perception of thirst and through sodium and water retention by kidneys.