Water and electrolyte balance is crucial for body homeostasis and is one of the most protected physiological mechanisms in the body. Water is vital for human existence. Water is colorless, calorie less compound of hydrogen and oxygen that virtually every cell in the body needs to survive. This chapter covers the water requirement and regulation, water metabolism and distribution, electrolytes, types, sources and consumption of body fluids, maintenance of fluid and electrolyte balance, dehydration, over hydration and water intoxication, and electrolyte imbalance. Electrolytes are substances whose molecules dissociate or split into ions when placed in water. These substances are found in ECF and ICF that dissociate into electrically charged particles known as ions. Human body contains quite large volume of water as ICF and ECF and the fluid contains several inorganic ions such as sodium, potassium, chloride, bicarbonate, sulfate, phosphate, calcium and magnesium.