This volume concerns the proper site-specific selection of appropriate technology for water treatment and waste disposal systems in cities and towns of LDC's. Many water projects have not worked when direct technology transfers have resulted in the selection of treatment processes too sophisticated or costly for in country construction, maintenance, or operation. Chapters I and II outline the difficulties which occur in donor/client relationships and technology transfer. Chapter III explains a methodology for selecting the most appropriate technology for water and wastewater treatment for a specific LDC site and at a particular time, according to the material and manpower resources available. Chapter IV presents a mathematical model for LDC's in Africa, Asia, and Latin America to predict water and wastewater demand, as well as construction, operation, and maintenance cost estimates for slow sand filters, rapid sand filters, stabilization lagoons, aerated lagoons, activated sludge systems, and trickling filters. The model used step-wise multiple regression, working from LDC in country data. Chapter V gives a methodology for setting priorities among water supply programs. Chapter VI-X give state-of-the-art resumes on past, present, and future technologies for water and wastewater for application in LDC's; these include on-site disposal and treatment concepts.