Copeptin is secreted in equimolar amount to Arginine Vasopresin (AVP) but can easily be measured with a sandwich immunoassay. Both peptides, copeptin and AVP, show a high correlation. Accordingly, copeptin mirrors the amount of AVP in the circulation and its measurement provides an attractive marker in the diagnosis and differential diagnosis of vasopressin-dependent disorders of fluid homeostasis. This talk will first give some background about copeptin in general and then highlight the use of copeptin for the diagnosis and differential diagnosis of vasopressin-dependent fluid disorders, i.e. Diabetes insipidus on one hand and the syndrome of inappropriate antidiuretic hormone secretion (SIADH) on the other hand.