Acute gastroenteritis is a public health problem of global proportions. Worldwide there are more than 700 million cases of diarrheal disease resulting in around 5 million deaths. In the United States the incidence of acute gastroenteritis is 11% each year and it is second only to respiratory infection as a contagious disease in the American family. The etiology of this common syndrome is obscure but it is becoming apparent that much of this disease is viral in origin. Recent research has elucidated a growing number of human pathogens: Norwalk-like agents, adenoviruses, caliciviruses, and astroviruses. The best understood of these agents are among the earlier discoveries in gastrointestinal virology—the rotaviruses. The study of these agents has greatly advanced our understanding of viral gastroenteritis.