The need for faster diagnosis, more accurate prognostic assessment and treatment decisions in various diseases has lead to the investigations of new biomarkers. The hope is that this new biomarkers will enable early decision making in clinical practice. Arginine vasopressin (AVP) is one of the main hormones of the hypothalamic-pituitary-adrenal axis. Its main stimulus for secretion is hyperosmolarity, but AVP system is also stimulated by exposure of the body to endogenous stress. Reliable measurement of AVP concentration is difficult because it is subject to preanalytical and analytical errors. It is therefore not used in clinical practice. Copeptin, a 39-aminoacid glycopeptide, is a C-terminal part of the precursor pre-provasopressin (pre-proAVP). Activation of AVP system stimulates copeptin secretion into the circulation from the posterior pituitary gland in equimolar amounts with AVP. Therefore, copeptin directly reflects AVP concentration and can be used as surrogate biomarker of AVP secretion. Even mild to moderate stress situations contribute to release of copeptin. These reasons have lead to a handful of research on copeptin in various diseases. This review summarizes the current achievements in the research of copeptin as a diagnostic and prognostic marker and also discusses its association in different disease processes.