This paper addresses the growing magnitude of morbidity and mortality from diarrheal disease; the definition, causes, and impact of chronic diarrhea; and unanswered questions about microbial etiologies, determinants of host susceptibility and impact, and the relevance of sociocultural setting to control strategies. Over 7% (12,600 children/day) of children in Africa, Asia, and Latin America die from diarrhea in their first 5 years of life, and poverty and rapid population growth are contributing to a worsening of this situation. Although advances have been made in our understanding of the etiology and management of acute diarrhea, little is known about the problem of chronic diarrhea the illness associated with the greatest risk of malnutrition. Moreover, 30-50% of cases of diarrhea remain with undiagnosed etiologies. Further study is needed to determine the role of cellular and humoral immunity in the specific enteric infections in order to enhance this immunity in new vaccine development. Genetic and age determinants of host susceptibility are important problems requiring further study. The social setting in which enteric infections take place may contribute to the transmission of or susceptibility to enteric infections. Future approaches to the control of diarrheal diseases must integrate advances in the following areas: epidemiologic, physiologic, biochemical, pharmacologic, immunologic, and antimicrobial control. The modern tools of molecular biology and improved understanding of scientific and social issues are expected to usher in an exciting new era in research on diarrheal diseases.