Examination of data on the availability of adequate water supplies in developing countries reveals an alarming state of affairs. This paper presents an analysis of the potential impact of a water supply in a poor tropical community and examines techniques for the quantification of this impact. Special attention is paid to the impact of a supply on the water collection journey (time and energy saving) and to the anticipated improvements in community health. A classification of water-related disease is presented and the effect of water supplies on various water-related infections is analyzed. A comprehensive table, showing the principal features of the water-related diseases, is included. It is shown how the analysis of water supply impacts, and in particular impacts on water collection and health, can facilitate improved designs and more rational resource allocation for water supply programs in developing countries.