Hepatitis Delta virus (HDV) is an incomplete virus dependent on the hepatitis B virus (HBV) for its replication. Serologic studies have shown that HDV is found all over the world. However, the prevalence is not simply a function of the prevalence of HBV, but HDV has an epidemiology of its own with major geographic differences as exemplified below. A high rate with as many as 60-80% anti-HDV positives among chronic HBsAg-carriers has been found in the northern part of South America, Central Africa and Asiatic Russia. In Europe, USA and Australia the prevalences of anti-delta among chronic HBV carriers in the general population are low. It is high, however, in special risk groups such as drug addicts. In the Far-East a very low frequency of anti-HDV is found. There are two different types of non-A, non-B hepatitis, transmitted either enterically or parenterally. The different agents are now partly characterized. The agent causing enterically transmitted disease has characteristics comparable with calici virus. The designation hepatitis E virus has been proposed. Epidemics of hepatitis caused by this agent has been found in three continents. The outbreaks have been associated with fecal contamination of drinking water. The agent causing parenterally transmitted non-A, non-B hepatitis has been reported to be partly defined by molecular hybridization techniques and is probably related to flavi virus. The name hepatitis C virus has been proposed. Parenterally transmitted non-A, non-B is predominantly seen in recipients of blood or blood products and among drug addicts.