Viruses are one of the most frequent causes of acute infectious gastroenteritis. Thus, rotaviruses are responsible for 40-60%--and in winter sometimes for over 90% - of diarrhoea in infants. These viruses may also cause disease in adults, but small viruses of a size of about 27 nm, such as the Norwalk agent, play a much more important role. Several other non-cultivable viruses such as adeno-, calici- and astroviruses have been implicated as a cause of infectious diarrhoea. Knowledge of rotaviruses is so far advanced that routine laboratory diagnosis can easily be performed and active immunization against rotavirus infection envisaged.