The terms mole, molality, molarity, osmole, osmolality, osmolarity, osmolar gap and anion gap are defined and their clinical usefulness indicated. The following body fluid compartments are described: total body water ( TBW), extracellular fluid ( ECF), intracellular fluid ( ICF), transcellular fluid ( TCF), plasma volume, red cell volume and interstitial fluid volume. Isotope-dilution techniques are briefly discussed and representative normal values for the various compartments according to sex and age are indicated. The physiological mechanisms that maintain the distinctive ionic compositions of the various fluid spaces are briefly outlined. New concepts of the function of the gel matrix and of the lymph drainage of the interstitium are presented. Opposing models to the sodium-potassium membrane pump are briefly described.