Norwalk and Norwalk-like viruses are the most common cause of foodborne viral illness. Outbreaks of “food poisoning” or foodborne gastroenteritis over the years have been assumed to be caused by bacteria, whether or not such organisms are identified. In many incidents, no traditional food poisoning organism is found, and many notable incidents of unknown etiology have been recorded. It was the application of electron microscopy and the discovery of several gastroenteritis viruses in the 1970s that opened up the possibility of investigating apparent food poisoning outbreaks in which food poisoning bacteria were not detected. Viruses causing gastroenteritis usually are transmitted directly from person to person by the fecaloral route, but it now is recognized clearly that on occasions some also may be transmitted through contaminated food and water. Of these, it is the Norwalk group of viruses that is the most important. Other gastroenteritis viruses rarely are involved.