Hyponatraemia, defined as a serum sodium concentration <135 mmol/L, is the most common disorder of body fluid and electrolyte balance encountered in clinical practice. Hyponatraemia is present in 15–20 % of emergency admissions to hospital and occurs in up to 20 % of critically ill patients. Symptomatology may vary from subtle to severe or even life threatening. Despite this, the management of patients remains problematic. Against this background, the European Society of Intensive Care Medicine, the European Society of Endocrinology and the European Renal Association–European Dialysis and Transplant Association, represented by European Renal Best Practice have developed a Clinical Practice Guideline on the diagnostic approach and treatment of hyponatraemia as a joint venture of three societies representing specialists with a natural interest in hyponatraemia.