This chapter describes the function of fluids and electrolytes in the body and discusses the functional components of fluids and electrolytes. It explains the Systeme Internationale (SI) units used in the measurement of concentrations and volumes of fluids and electrolytes in the body. The mole is the unit that is used to measure the amount of molecules. Osmolality is a measure of the concentration of osmotically active particles per kilogram of the solvent. Valency is a measure of an atom's combining power with other atoms when it forms chemical compounds or molecules. The chapter differentiates the major intracellular and extracellular electrolytes. The intracellular and extracellular electrolyte differential is maintained in normal human physiology by both active and passive processes, and key to maintaining this differential is the point at which electrolytes are transported across cell membranes by active processes. The chapter also discusses the overall regulation and control of fluid and electrolyte balance.