U.S. government support for water supply and sanitation (WS&S) projects in Guatemala was initiated in 1943 by the Inter-American Cooperative Service for Public Health (SCISP). Except for a brief period during the 1950's, U.S. support for the sector has been continuous and has grown to include both urban and rural areas. Overall, A.I.D.'s investments in water systems in both urban and rural areas and for sanitation systems in urban areas have been sustained, although those in urban areas seem to have been more successfully sustained than those in rural areas. The rural sanitation (latrine) programs have been the least sustainable. The urban WS&S projects owe their success to both contextual factors and project characteristics. Among contextual factors, a high level of national commitment to project goals, the sequencing of donors assistance, which provided a steady flow of funds, and the quality of the implementing institution, the National Institute for Municipal Development (INFOM), which also provides assistance to the municipalities that built systems under SCISP, have influenced the sustainability of urban projects. The key project characteristics important to project sustainability have been financing (including cost-recovery), institutional organizations and management, and project effectiveness. The essence of the urban situation is that the municipalities have been able to operate their systems because they receive good TA and financing from INFOM, with IDB support. In the rural sector, the most important contextual factors that have favorably influenced project sustainability are high national commitment and sociocultural influences (strong, indigenous community organization). Project characteristics that have been the most important to project sustainability are financing and the use of appropriate technology. The use of gravity systems, with their minimal maintenance requirements, permits the decentralization of the operations and enables communities to maintain their system without central government support. However, since future systems will have to be more complex, government support for system maintenance is likely to become more important. Although rural WS&S projects are affected by the same factors, these factors can operate in opposite ways. For instance, sociocultural factors have strengthened community capacity to maintain water systems but have worked against the acceptance of latrines. Moreover, while projects have applied an appropriate technology for water systems, they have not yet produced a latrine design that is widely accepted in rural areas. Finally, it is worth noting that during the course of this study, a number of Guatemalans pointed to the lack of A.I.D. involvement in sector coordination and institutional development as a critical shortcoming. (Author abstract, modified).