More than 14 million households nationwide depend on their own well, spring, or cistern to supply water for home use. Groundwater supplies more than 95% of domestic rural water needs. Groundwater also may provide water for livestock or irrigation. The quality of groundwater can be threatened by private sewage disposal systems as well as past and present industrial, municipal, and agricultural practices. Natural processes also affect water quality. Water testing is the only way to be sure that a water supply is safe. Individuals using public water supplies pay for water testing and treatment as part of their water bills. In most states, regulations mandate neither water testing nor treatment of private water supplies. Some states mandate water testing when a new well is drilled or when property is sold. Individuals supplying their own water must arrange for water testing; if their water has unsafe or undesirable characteristics, they must take the initiative to correct the problem. Individuals supplying their own water must be aware of potential dangers to their water supplies and of methods to assure the supplies are safe. This bulletin reviews the currently accepted standards for drinking water safety and domestic use. It provides information on testing water for contamination as well as on land use practices that may affect water quality. The bulletin also reviews options for improving the quality of problem waters or for developing new supplies. Appendix C contains information on water quality for livestock and poultry.