Hyponatremia is a common laboratory finding in numerous patients. It is defined as a serum sodium concentration <135 mmol/L and represents an excess of water in the extracellular compartment. The severity of this electrolyte abnormality ranges from asymptomatic to seizures, coma and death as a consequence of cerebral swelling. There are multiple medical conditions, medications and disease states that can cause hyponatremia. This article summarizes the important pathophysiological pathways involved in the development of hyponatremia, describes an approach to common causes and reviews the initial steps in management.