Summary. Chronic delta hepatitis (CDH) represents a severe form of chronic viral hepatitis, induced by the hepatitis delta virus (HDV) in conjunction with the hepatitis B virus (HBV). Delta hepatitis may lead to disease in humans through co-infection. The former leads to acute hepatitis which clinically can range from mild hepatitis to fulminant hepatitis and death. Severe or fulminant hepatitis is more often observed with HBV-HDV co-infection compared to HBV mono-infection. Chronic infection after acute hepatitis B + D co-infection is infrequent and similar to the rate in mono-infected patients. CDH develops in 70–90% of patients with superinfection. CDH runs a more progressive course than chronic hepatitis B and may lead to cirrhosis within 2 years in 10–15% of patients. However, as with any immune-mediated disease, different patterns of progression, ranging from mild to severe progressive disease, are observed. Active replication of both HBV and HDV may be associated with a more progressive disease pattern. Further, different HDV and HBV genotypes may contribute to various disease outcomes. CDH may be frequently associated with hepatocellular carcinoma development although recent studies provided conflicting results. The only established therapy for CDH is treatment with interferons for a duration of at least 1 year. On treatment, 6 month HDV RNA assessment may give clues as to whether to stop treatment at 1 year or continue beyond 1 year. New approaches to treatment of CDH are an urgent need of which the use of prenylation inhibitors appears the most promising.