Some 10 years of experience in water supply and sanitation (WS&S) projects in 60 countries under USAID's Water and Sanitation for Health (WASH) Project are distilled into "lessons learned" in this report. The lessons -- 19 general lessons and 52 more specific ones -- are addressed in some instances to funding agencies, and in others directly to project planners and implementers. Thematically, four topics are covered. The first set of lessons deals with establishing a technical assistance program and is particular relevant to donors and central governments. Lessons in this category stress the importance of institution building, interdisciplinary work, a participatory approach, coordination and collaboration, and an active information service. The second set of lessons deals with linkages, particularly the importance of linking WS&S projects to hygiene education, which in turn leads to behavioral changes on the part of the beneficiary community and enhances health benefits. Next, lessons for achieving system sustainability focus on the areas of institutional development, training, appropriate engineering design, and early planning for operations, maintenance, and financing. The final set of lessons addresses the issue of shared responsibility, outlining the respective roles of national governments, donors, NGO's, the community, and private enterprises.