On February 28, 2002, the US Environmental Protection Agency (USEPA) promulgated the new Arsenic Rule and set the Maximum Contaminant Level (MCL) for arsenic at 10 ug/L. This lower arsenic MCL will result in many groundwater systems needing to treat their water supplies to comply with this rule. The USEPA estimates that approximately 3,300 water systems will be impacted by the new Arsenic Rule, the overwhelming majority of these systems are small groundwater systems serving less than 3,300 people. There are several technologies that the USEPA has identified as Best Available Technologies (BAT) for arsenic removal including ion exchange, activated alumina, reverse osmosis, electrodialysis reversal, and coagulation/filtration. However, in the Arsenic Rule, the USEPA recognized an emerging technology pioneered by CH2M HILL in 1994 (Chang, 1994) for arsenic removal, the coagulation/microfiltration (C/MF) process. Prior to and during the Arsenic Rule development, the USEPA and others suggested that the C/MF process was too expensive and complex for small systems, and that only a few large water systems would consider this technology. However, the cost of membranes, microfiltration in particular, continues to decrease and become competitive with other technologies. In addition, the water industry has gained significant operational experience with microfilters over the last 10 years, resulting in a greater level of comfort about their complexity. The C/MF process has been pilot tested at several locations for the removal of arsenic including Albuquerque, New Mexico (Chwirka, 2000), Fallon, Nevada, and El Paso, Texas, and Phoenix, Arizona (Amy, 2000). This paper presents a fundamental background related to the C/MF process for arsenic removal and provides a summary of pilot testing that has been performed at several sites. Includes 4 references, figures.