Nurses need to have a firm grasp of normal and pathophysiological mechanisms of sodium and water balance to fully understand assessment findings and establish a rationale for a patient's plan of care. While multiple mechanisms control sodium and water balance, antidiuretic hormone and aldosterone are the most important hormonal influences. This article, the first in a new series designed to improve nurses' understanding of the physiological abnormalities underlying many disorders, reviews the common etiologies and symptoms of hyponatremia and hypernatremia, as well as the role of nursing care in patients with imbalances of sodium and water. Case studies guide the reader through relevant medical history and examination findings to an understanding of both the nursing and medical plans of care.