Copeptin, arginine vasopressin (AVP)-associated 39 aminoacid glycopeptide, is a C-terminal part of pro-AVP. AVP acts through V1a, V1b, and V2 receptors. The effect on V1a receptors is connected with arterial vasoconstriction, on V2 with antidiuretic action, and on V1b with the secretion of ACTH, insulin, glucagon. Copeptin is found in the circulation in equimolar amounts with AVP. It is a very stable peptide and easy to estimate. Copeptin is a good diagnostic marker in many disorders in which vasopressinergic dysfunction plays a role in pathogenesis such as a polyuria-polydipsia syndrome, neurological disease (ischemic stroke, nontraumatic, intracerebral hemorrhage, aneurysmal subarachnoid hemorrhage and neurodegenerative disease (multiple sclerosis). Copeptin is a diagnostic and prognostic marker in cardiovascular diseases like heart failure (HF) and acute myocardial infarct (AMI). Copeptin is a sensitive diagnostic marker in the early stage of AMI especially in patients with non-ST segment elevation and post AMI complications. Copeptin is also an important diagnostic and prognostic marker in metabolic diseases (diabetes mellitus, metabolic syndrome, insulin resistance), connected with some neurological and cardiovascular diseases. In the future, these findings may have also therapeutic applications in conditions where the AVP receptor antagonist therapy is appropriate.