Disturbance of sodium balance is a finding in the hospital inpatient population and is common in critically ill patients and after brain injury. An understanding of the normal physiology of sodium homeostasis allows a systematic approach to the diagnosis and treatment of dysnatraemia. Sodium and water homeostasis are inextricably linked. Sodium is the major extracellular cation and with its accompanying anions, principally chloride and bicarbonate, is the major determinant of plasma osmolality. Hyponatraemia is classified into hypovolaemic, euvolaemic or hypervolaemic types depending on the associated volume disturbance. Hyponatraemia reflects an excess of water relative to sodium either because of dilution of sodium by increase in total body water or depletion of total body sodium in excess of simultaneous water losses. The syndrome of inappropriate anti-diuretic hormone secretion (SIADH) is one of the most commonly encountered causes of hyponatraemia in hospital inpatients and in the critically ill.