Molecular biology techniques are now a vital part of hepatitis virology, with a central role in studies of diagnosis, epidemiology, virology, pathogenesis, and natural history of infection. Cloning of the genome of hepatitis E virus has allowed its tentative classification as a calici- or related virus, and is the first step toward the development of a vaccine. Long-term implications of hepatitis C for groups such as children with hemophilia, thalassemia, and even leukemia can be better understood by comparison of virus load measured by molecular amplification of the plasma viral RNA with the serologic and clinical status of the respective cohorts of children. A new vaccine for hepatitis A has been licensed in several European countries, and recent experience with severe hepatitis in infants after unexpected transmission of hepatitis B from anti-hepatitis B e positive mothers reemphasizes the value of universal hepatitis B immunization programs. Mother-to-infant transmission of hepatitis C virus has now been well documented, but there are still insufficient data on the dynamics of this, particularly in the absence of passive immunoprophylaxis or a vaccine, to permit recommendations regarding the management of individual pregnancies or deliveries. There is especially too little information to suggest whether breast feeding may be an important mechanism for transmission.