This study sought to assess the impact of inadequate water and sanitation on diarrheal disease in low- and middle-income settings. Overall, the authors found that improvements in drinking water and sanitation were associated with decreased risks of diarrhea. Specific improvements, such as the use of water filters, provision of high-quality piped water and sewer connections, were associated with greater reductions in diarrhea compared with other interventions. Results show that inadequate water and sanitation are associated with considerable risks of diarrheal disease and that there are notable differences in illness reduction according to the type of improved water and sanitation implemented.