The design of water treatment plants for developing countries should reflect the prevailing social, economic, and technical conditions. A simple plant is far more likely to remain operable over a period of years than a duplicate of a plant in a developed country. The following technologies, illustrative of the kinds of treatment appropriate for communities in developing countries, are discussed: pretreatment; chemicals; chemical feeders; hydraulic rapid mixers; hydraulic flocculators; horizontal-flow settling basins; interfilterwashing filtration units; direct filtration; upflow-downflow filters; slow sand filters; and interfilter-washing declining-rate filters. Indigenous materials are also more suitable where feasible. For example, coagulants and coagulant aids can be derived from seeds or the exoskeletons of arthropods if alum is not locally available. Includes 7 references, table, figures.