ATSDR’s Geospatial Research, Analysis & Services Program (GRASP) has created a tool to help public health officials and emergency response planners identify and map the communities that will most likely need support before, during, and after a hazardous event. The Social Vulnerability Index (SVI) indicates the relative vulnerability of every U.S. Census tract. Census tracts are subdivisions of counties for which the Census collects statistical data. The SVI ranks the tracts on 15 social factors, including unemployment, lack of vehicle access, and crowded housing, and further groups them into four related themes. Thus each tract receives a ranking for each Census variable and for each of the four themes, as well as an overall ranking.