Disorders of electrolyte balance are frequent and pathophysiologically complex. Sodium is responsible for a large part of the osmolarity of extracellular fluids. Therefore, pathological concentrations of serum sodium reflect the relation between sodium and water in the extracellular compartment rather than the total body sodium content. The causes of hypo- or hypernatremia can only be deduced if total body volume status is considered. Patients with hyponatremia and volume deficit should receive sodium chloride solution while patients with this disorder in the presence of volume overload need strict water restriction. In certain cases additional specific pharmacotherapy directed at the effects of antidiuretic hormone may be considered. Potassium and calcium are extracellular regulatory ions; their concentrations do not relevantly contribute to osmolarity and water distribution but to electrophysiologically relevant transmembrane potentials. These ions are influenced by active membrane transporters and regulated by several hormones. The rather small extracellular pools are overfilled or depleted by alterations of intake and excretion. In addition, several inborn or acquired defects of transmembrane transporters may severely alter their extracellular concentrations. Therapy needs to consider the specific mechanisms that led to the electrolyte disorder including modification of intake, excretion or extra-intracellular distribution.