The main literature concerning the physiology and biochemistry as well as the pathophysiology and pathobiochemistry of magnesium is reviewed, including: Distribution and physico-chemical state of magnesium in the extracellular and intracellular fluid as well as in the subcellular organelles (membranes, mitochondria, microsomes, ribosomes). Intestinal resorption, transport across membranes and excretion by the kidney. Hormonal regulation of magnesium distribution and its clinical disturbances. Biochemical mechanism and the clinical effects of hypo- and hypermagnesemia.