<p>Osmolality disorders are disturbances in the water balance. When the amount of water increases relative to the amount of solutes or osmoles, the osmole concentration decreases. In contrast, when the amount of water decreases relative to the amount of osmoles, the concentration increases. As sodium is the main osmolyte in the extracellular space, disturbances in water balance manifest as changes in the plasma sodium concentration. In the clinical practice osmolality disorders (water) must be distinguished from volume disorders (salt) and combinations of both disorders must be recognized. Hypotonic hyponatremia is the most frequent osmolality disorder and even the most frequent electrolyte disorder overall. Knowledge of the causes, clinical symptoms and treatment strategies is essential for the optimal management of patients with hyponatremia.</p>