The primary goal behind the development of a GIS for the U.S. EPA's Acid Rain Program is to gain a thorough understanding of the impacts of sulfur dioxide and nitrogen oxide emission reductions to the environment. The decision to implement a GIS stemmed from a need to better understand the implications of policy questions regarding acid rain control and therefore to improve the decision making process. GIS is a unique analysis tool to support this effort by visually integrating environmental, ecological, and other data to assess the overall impacts of emission reductions that are national or regional in nature.