Many people in developing countries rely upon untreated groundwater supplies for their drinking water. These supplies are obtained from drilled boreholes or tubewells, dugwells and springs. Such sources are usually of good quality and much better than some traditional sources of supply, for example ponds and streams. However, groundwater can become contaminated and there is special concern that the introduction of on-site sanitation systems may in certain circumstances contribute to contamination of drinking water supplies. The purpose of this manual is to provide guidance on how to assess and reduce the risk of contamination of groundwater supplies from onsite sanitation systems and is aimed at those responsible for planning low cost water supply and sanitation schemes.