Copeptin, a C-terminal part of the precursor pre-provasopressin is a novel biomarker of arginine-vasopressin (AVP) system. Measurements of AVP concentration are not used in clinical practice because of technical difficulties. Copeptin is synthesized in stoichiometric ratio with AVP, hence it reflects vasopressin concentration in human plasma and serum. This review outlines current research concerning the role of copeptin as a prognostic marker in different diseases and its potential clinical value.