In studying the causes of human gastroenteritis, electron microscopy and related techniques have led to the identification of new viral agents that had previously escaped detection by routine cell culture procedures. Efforts to characterize and study these agents further are currently being made by researchers in many areas of the world. Two of the best‐known agents, rotavirus and Norwalk virus, have been implicated in waterborne outbreaks of this illness. Another virus, the Snow Mountain agent, was first identified from the investigation of one waterborne gastroenteritis outbreak.