Much has been learned in the past three to five years about the causes of infectious enteric disease and about the pathogenesis of infection with some of the causative agents. However, many questions remain on the ecology ad epidemiology of these agents, the pathophysiology of infection and factors in resistance. Some of the answers require selective cross-sectional and longitudinal studies. We propose: **To characterize clinical illness in relation to infecting agent in an outpatient HMO population, a neonatal nursery population and in patients hospitalized for gastroenteritis. **To determine the age-specific incidence of diarrhea and its causes in a well-defined population. **To define the magnitude and future impact of rotaviral infection and illness in neonatal infants. **To study factors in transmission. **To define factors in immunity and resistance. **To determine the role of rotaviruses and other agents in infants and children who die. **To develop, evaluate and validate certain laboratory methods.