Hypernatremia is generally defined as serum sodium above 145mEq/L. It is known that about 3% of hospitalized patients develop hypernatremia. In addition, it has been reported that hypernatremia develops in about 9% of patients admitted to the intensive care unit and is associated with high mortality. Hypernatremia is most often caused by an inability to replace water loss due to impaired thirst or lack of access to water. It can also be caused by excessive salt intake or administering a hypertonic salt solution. The cause of the hypernatremia is usually evident from the history and physical examination. If the etiology of hypernatremia is unclear from history alone, the cause can be inferred through various blood and urine tests, and in rare cases, genetic testing is also required. The treatment of hypernatremia involves treating the underlying cause and correcting the water deficit. In this lecture, the results of recent studies on the relationship between BPV and renal outcomes will be summarized, and the differences and limitations will be explored. In this lecture, we will evaluate various causes from common to rare condition of hypernatremia and polyuria through case studies, and cover the process of treating hypernatremia.