HAV is a small, non-enveloped, and single-stranded RNA virus. The virus is transmitted from person to person through the fecal-oral route and through the ingestion of contaminated food. HAV infection normally affects children without producing symptoms but in adults it causes clinically apparent disease. The virus can spread easily from asymptomatic children to other children and to adult contacts. Children are thus considered to be a principal reservoir and dominant source of transmission in the community. Infection is maintained by serial transmission from infected individuals to those who are susceptible. While almost acute hepatitis A patients subside without progress to chronic liver disease, some patients may have relapsing, prolonged, cholestatic and fulminant hepatitis. Hepatitis A vaccine has been used successfully to prevent hepatitis A infection worldwide since mid-1990s. The sero-positivity pattern of anti-hepatitis A virus antibody (anti-HAV) has been changing rapidly from the intermediate to the low during the past 30 years in Korea. Thus adulthood acute hepatitis A became widespread in last 10 years in Korea due to decreased seroprevalence of anti-HAV. It can be recognized that acute hepatitis A is a "growing and living", not a "fading and dying" disease. The changes in seroepidemiologic pattern may cause sporadic and large community outbreaks and lead to increased healthcare cost. In this situation, we should have our own vaccination strategy to prevent and future eradication of hepatitis A infection.