Hyponatraemia is the most common electrolyte disbalance in clinical practice, which is associated with increased patients morbidity and mortality. At present the pathophysiology of hyponatraemia is explored in more details, antidiuretic hormone and osmoregulation play the major roles. This article informs about relatively new classification of hyponatraemia for clinical practice based on the severity of clinical symptoms and based on the effective serum osmolality. It also offers diagnostic and treatment guidelines of hyponatraemia, which are based on current recommendations of the world experts and on the evidence based medicine.