Elevated concentrations of arsenic (>10 ppb) are common in groundwater throughout the southwestern United States and northern Mexico. These arid regions largely depend on groundwater as their sole or primary drinking water source. Primarily the arsenic originates from natural geological deposits, although anthropogenic sources, particularly mining and industrial wastes, may also contribute. Based on recent toxicological studies indicating unacceptably increased cancer risks below 50 ppb of arsenic, the United States lowered its arsenic in drinking water standard to 10 ppb in 2001 which brings it into line with the World Health Organization recommended standard. This more stringent limit is causing over 250 water providers in Arizona alone to reduce arsenic levels in their delivered water. Most are implementing some form of arsenic removal technology. However, the choice of the appropriate technology for a particular application is extremely case specific due to the breadth of modes of action of the available technologies, the widely varying source water compositions, and the presence or absence of co-contaminants of concern. In this talk, the underlying chemistry controlling the efficacy of arsenic technologies is discussed in relationship to the range of water compositions and co-contaminants encountered in field conditions. In many cases, pre-adjustment of the source water chemistry (e.g., pH adjustment, selective ion removal) may be indicated prior to the actual arsenic removal process. In addition, the pivotal role that chemistry plays in arsenic drinking water treatment extends beyond the removal process into the treatment residual management realm. Due to redox and colloidal chemistry issues the arsenic-bearing waste residuals from the most widely chosen, arsenic removal treatment processes are often unstable under normally accepted waste disposal conditions and consequently may release their arsenic back into the aqueous phase. The chemistry controlling these releases will also be discussed.