Viral infections of the gastrointestinal tract continue to be a major worldwide cause of morbidity and mortality, especially in underdeveloped settings. Rapid improvement in the understanding of viral structure and function, epidemiology, and host immune responses are occurring in conjunction with advances in basic science technologies (monoclonal antibodies, molecular genetic techniques, polymerase chain reaction, and so on). Intestinal adenoviruses, astroviruses, caliciviruses, Norwalk and Norwalk-like viruses, and atypical rotaviruses are all joining Group A rotavirus as well-described human pathogens, although rotavirus continues to be recognized as the leading cause of severe gastroenteritis in children.