This manual and the free downloadable costing tool is the outcome of a project identified by the Water, Sanitation and Health Programme (WSH) of the World Health Organization (WHO) faced with the challenge of costing options for improved access, both to safe drinking water and to adequate sanitation. Although limited in scope to the process of costing safe water supply technologies, a proper use of this material lies within a larger setting considering the cultural, environmental, institutional, political and social conditions that should be used by policy decision makers in developing countries to promote sustainable development strategies.