A great progress in the knowledge about hepatitis virus has occurred in the last decades and several virus have been identified. Virus B, C and D are transmitted parenterally, while virus A and E use the enteral route. Highly effective vaccines for A and B hepatitis are presently available. Virus C is an important cause of chronic liver disease at the present moment. Since the identification of virus C and E, the number of acute hepatitis denominated as non A non B has decreased considerable. Although there are still patients with viral hepatitis of unknown origin and there is considerable effort to identify the agents causing them. Virus G and TT are frequently present in the sera of patients with chronic liver diseases but their real pathogenic role is not completely elucidated.