Hepcidin, which has been recently identified both by biochemical and genomic approaches, is a 25 amino acid polypeptide synthesized mainly by hepatocytes and secreted into the plasma. Besides its potential activity in antimicrobial defense, hepcidin plays a major role in iron metabolism. It controls two key steps of iron bioavailability, likely through a hormonal action: digestive iron absorption by enterocytes and iron recycling by macrophages. In humans, this could explain that low levels of hepcidin found during juvenile haemochromatosis and HFE-1 genetic haemochromatosis are associated with an iron overload phenotype. Conversely, an increase of hepcidin expression is suspected to play a major role in the development of anemia of chronic inflammatory diseases. However, the regulatory mechanisms of hepcidin expression are multiple, including iron-related parameters, anemia, hypoxia, inflammation and hepatocyte function. Therefore, many physiological and pathological situations may modulate hepcidin expression and subsequently iron metabolism. A better knowledge of the biological effects of hepcidin and of its expression regulatory mechanisms will clarify the place of hepcidin in the diagnosis and treatment of iron-related diseases.