The Arsenic Cost Working Group of the National Drinking Water Advisory Council has completed its analysis for the cost of implementation of the arsenic rule. This report includes the group's findings and recommendations. The charge of the National Drinking Water Advisory Council (NDWAC) Arsenic Cost Working Group was to review the costing methodologies, assumptions, and information underlying the system-size cost estimates as well as the aggregated national estimate of system costs of the Arsenic in Drinking Water Rule. As part of this review, the working group was to evaluate alternative costing approaches or critiques that may have a significant impact on the estimated system costs. In making this evaluation, the working group was charged to determine whether there is adequate supporting information upon which to evaluate the basis for the alternate approaches or critiques and note where there is not adequate supporting information. The final element of the working group's charge was to develop, based on its review and analysis, a written recommendation to the NDWAC. Such recommendations are to be provided with the understanding that EPA will decide whether and how any revision of the arsenic rule's cost analysis will occur.