When the critically ill patient's clinical course is complicated by diabetes insipidus, a significant threat to the equilibrium of the body's delicate water and electrolyte balance ensues. Patients with diabetes insipidus are at significant risk of dehydration, hypernatremia, alterations in level of consciousness, and hemodynamic instability from hypovolemia. This article presents the causes of, and pathophysiologic alterations in, central and nephrogenic diabetes insipidus. Clinical manifestations and therapeutic management of both conditions in the critical care population are described.