- There are several regions worldwide with a high prevalence of infection with the hepatitis delta virus (HDV) in hepatitis B virus (HBV) carriers.- Chronic HDV infection is occurring with increasing frequency due to increased immigration.- HDV transmission can take place through the same routes as HBV by simultaneous infection with both viruses (co-infection) or infection of an HBV carrier with HDV (superinfection).- Delta hepatitis is considered as the most severe form of viral hepatitis with a high risk of progression to cirrhosis and hepatocellular carcinoma.- Chronic delta hepatitis is exclusively observed in patients who are HBV carriers.- Pegylated interferon is currently the only registered therapy for patients with delta hepatitis, but leads to a persistent virological response in only a minority of them, and rarely leads to a complete cure.- New antivirals, such as viral entry blockers, prenylation inhibitors and anti-sense oligonucleotides are promising and currently being investigated in phase 2 trials.