As the human population grows--tripling in the past century while, simultaneously, quadrupling its demand for water--Earth's finite freshwater supplies are increasingly strained, and also increasingly contaminated by domestic, agricultural, and industrial wastes. Today, approximately one-third of the world's population lives in areas with scarce water resources. Nearly one billion people currently lack access to an adequate water supply, and more than twice as many lack access to basic sanitation services. It is projected that by 2025 water scarcity will affect nearly two-thirds of all people on the planet. Recognizing that water availability, water quality, and sanitation are fundamental issues underlying infectious disease emergence and spread, the Institute of Medicine held a two-day public workshop, summarized in this volume. Through invited presentations and discussions, participants explored global and local connections between water, sanitation, and health; the spectrum of water-related disease transmission processes as they inform intervention design; lessons learned from water-related disease outbreaks; vulnerabilities in water and sanitation infrastructure in both industrialized and developing countries; and opportunities to improve water and sanitation infrastructure so as to reduce the risk of water-related infectious disease.