Hypernatremia is potentially life threatening and is caused by imbalances in water and sodium that occur from either water loss or sodium gain, often in the presence of inadequate water intake. The resultant hyperosmolarity, if acute or severe, can result in rapid shifts of water from the intracellular to extracellular space, causing intracranial hemorrhage and neurologic changes. Correction of hypernatremia through inappropriate fluid therapy can result in even more severe complications, ranging from cerebral edema to coma and death. This article reviews the pathophysiology, causes, classification, treatment, and prognosis of hypernatremia.