Twenty lessons learned from the 13 years (1980-1993) of field work in the Water and Sanitation for Health (WASH) project are distilled in this report. in regard to TA, WASH teaches that: effective TA focuses on building local institutions by transferring sustainable skills; an active information service can expand the reach of TA; and, TA in water supply and sanitation (WS&S) requires an approach that is interdisciplinary and participatory, and the collaboration of donor agencies. Lessons concerning shared responsibility are that: government's role is to assume responsibility for overall sector management, that of donors to support the government's national plans; NGOs can operate effectively in local situations inaccessible to donors; user participation in WS&S management is critical to sustainability; expansion of the private sector's role depends on a supportive legal and policy environment. Lessons regarding program strategies are: the success of individual projects depends on strong sectoral policies and institutional practices; sanitation should be accorded the same priority as water supply; improvements in hygiene are an indispensable measure of success for WS&S activities; national governments must take specific policy steps to empower communities to manage WS&S efforts; a participatory approach to planning helps forge linkages beyond the sector; and, the traditional command and control model for WS&S regulation is generally inappropriate. Lessons for sustainability are that: successful institutional and human resource development projects are comprehensive, systematic, participatory, and based on long-term planning; sustainability of WS&S systems requires full consideration of appropriate engineering design and application, planning of operations and maintenance functions prior to construction to ensure that the technologies selected are sustainable, and an appropriate mix of donor, government, and community resources.