Hypernatremia is a common electrolyte disorder in daily clinical practice. In many cases hypernatremia is caused by a lack of free water or an increased salt load. Out-of-hospital acquired hypernatremia is often caused by an increased loss of water or a decreased water intake. By contrast, hospital-acquired, nosocomial hypernatremia is often induced by an inadequate fluid balance with saline infusions, saline overload, or due to osmotic diuresis. The consequences are structural changes in the cell morphology such as cell shrinkage. Chronic hypernatremia affects all cell functions predominantly with cerebral symptoms and coma; the main complication is a too-rapid compensation of an adapted electrolyte imbalance with development of cerebral edema. The overall osmolality should always be considered. Overall changes in osmolality correspond to the effect on the cellular stress situation and have to be taken into account and balanced slowly. In cases of unknown duration, a chronic disorder should be assumed.