Using household survey data, this paper estimates the mortality impact of improved water and sanitation access in order to evaluate the potential contribution of water and sanitation investment toward achieving the child mortality targets defined in Millennium Development Goal 4. The authors find that the average mortality reduction achievable by investment in water and sanitation infrastructure is 25 deaths per 1,000 children born across countries, a difference that accounts for about 40 percent of the gap between current child mortality rates and the 2015 target set in the Millennium Development Goals. According to the estimates, full household coverage with water and sanitation infrastructure could lead to a total reduction of 2.2 million child deaths per year in the developing world. Combining this analysis with cost data for water and sanitation infrastructure, the authors estimate that the average cost per life-year saved ranges between 65 and 80 percent of developing countries'' annual gross domestic product per capita. The results suggest that investment in water and sanitation is a highly cost-effective policy option, even when only the mortality benefits are taken into consideration. Taking into account the additional expected benefits, such as reduced morbidity, time spending, and environmental hazards, would further increase the benefit-cost ratio.