Acute viral gastroenteritis is an extremely common illness that affects all age groups throughout the world and occurs in both epidemic and endemic forms. Among illnesses affecting United States families, it is second in frequency only the common cold. The disease is usually self-limited, although it can be lethal to the malnourished, elderly, debilitated or infant patient. The two viral pathogens currently known to have medically important etiologic roles in human gastroenteritis, rotavirus and Norwalk virus, were discovered during the 1970's by electron microscopy. Development of rapid diagnostic immunoassays for these viruses has led to an understanding of their epidemiology and medical importance. This book chapter discusses the radioimmunoassay (RIA) and the enzyme immunoassay (EIA) procedures used to detect these viral agents.