Copeptin (also known as CT-proAVP) is a 39-amino acid-long peptide derived from the C-terminus of pre-pro-hormone of arginine vasopressin, neurophysin II and copeptin. The precursor protein is synthesized and cleaved in the hypothalamus before transport to the pituitary for storage and release. Copeptin is secreted in equimolar amounts with Vasopressin. The size and half-life of copeptin permit an easier immunological testing, compared to vasopressin, and hence copeptin is proposed as a reliable AVP surrogate. The clinical interest in copeptin testing is closely linked to the pathophysiological pathways in which vasopressin is involved: polydipsia-polyuria syndrome, hyponatremia, syndrome of inappropriate antidiuretic hormone secretion (SIADH) as well as heart failure and acute coronary syndrome.