Arsenic is a widely distributed, naturally occurring element in the Earth's crust and is present in trace amounts in all living organisms. Higher levels of arsenic tend to be found more frequently in ground water than in surface water. Because small water systems typically rely on wells for drinking water, while the largest systems typically rely on surface-water sources, arsenic tends to occur in higher levels more often in water used by small communities. In the United States, the average level measured in ground-water samples is less than or equal to 1 part per billion; however, higher levels are not uncommon. Compared to the rest of the United States, Western states have more water systems with levels exceeding 10 ppb, and levels exceed 50 ppb in some locations. Parts of the Midwest and New England also have some water systems with arsenic levels exceeding 10 ppb, but most systems have lower levels. EPA projects that 5.5% of water systems, serving 11 million people, are likely to exceed the 10 ppb level. Sources of arsenic in water include natural sources, and releases from its use as a wood preservative, in semi-conductors and paints, and from agriculture and mining. A question of ongoing scientific debate concerned whether significant adverse health effects occur from ingesting arsenic at very low levels. This book reviews EPA efforts to develop a new arsenic rule and summarises key provisions and subsequent events. Included are different ways of actually removing the arsenic and maintaining the healthy level that is required by the EPA.