Acute gastroenteritis due to Norwalk virus(NV), a member of the family Caliciviridae, is a common illness worldwide. NV is a human pathogen causing food-borne and water-borne diseases that occur in various epidemiological settings. The disease is mild and self-limiting and the symptomatic phase lasts 24 to 72 hours. Genetic analyses of the polymerase and the capsid protein indicate that NV is grouped into two clusters, genogroup I and genogroup II. The development of recombinant virus-like particles made a great impact on the development of diagnostic assays such as antigen ELISA and antibody ELISA. Strong control measures are needed to prevent oyster-related outbreaks.