Five viruses are responsible for the vast majority of cases of viral related hepatitis. They have been named hepatitis A virus (HAV), hepatitis B virus (HBV), hepatitis C virus (HCV), hepatitis D virus (HDV), hepatitis E virus (HEV). The more recent literature concerning the viral structure, the epidemiology, the serological identification, the clinical course and the prevention of each type of hepatitis is reviewed. HBV is not directly cytopathic. Hepatitis is a consequence of the destruction of the virus-infected cells. The efficient elimination of the virus relies on the viral antigenic determinants (HBs, pre-S1, pre-S2, HBc, HBe) and on the immune system of the host. The viral persistence may be caused by defect of the host immunity (interferon production, T and B lymphocyte function) or by factors related to the virus such as a genome mutation (lack of HBe formation). Some evidence suggesting an immunopathogenetic mechanism also for HCV, HDV and HAV is reported.