Diabetes insipidus (DI) is a syndrome characterized by chronic polyuria and polydipsia. It can result from any of three basic defects: (a) inadequate urinary concentration caused by a deficiency in the secretion or action of the antidiuretic hormone vasopressin (neurogenic or nephrogenic DI), or excessive intake of water caused by a defect in (b) thirst or (c) psychological function (dipsogenic or psychogenic DI). These four types of DI can be differentiated clinically only if they present in a complete and classical form. However, more sophisticated diagnostic approaches involving assays of plasma vasopressin or closely monitored trials of antidiuretic therapy usually are necessary when the patient has mild or incomplete defects in thirst or vasopressin function. Accurate diagnostic differentiation among the four basic types of DI is essential not only for safe and effective management but also for a proper understanding of the basic physiology and pathophysiology of water homeostasis.