Environmental epidemiology is an area of epidemiology concerned with the study of associations between environmental exposures and health outcomes, with the purpose of further understanding the etiology of disease. The term “environment” implies a spatial context. Thus, the study of interactions between humans and their environment requires spatial information and analysis. Geographic information system (GIS) software allows environmental and epidemiologic data to be stored, analyzed, and displayed spatially. The logical structure and functionality of a GIS are shown in Figure 1 (Falbo et al. 1991). Data collection can be accomplished by importing tabular or digital data that are referenced with map coordinates defining their geographic position. The data are entered into a database where they are stored as a map with a specified theme (termed “data layer”). Tabular (attribute) data corresponding to the theme can be stored with each data layer. Analytical functions within the software can be used to process and manipulate both map and attribute data through linkages established within the GIS. Two types of output are common: tabular (summary data, statistics, reports) and cartographic (maps, map files, and map overlays). Several publications describe the structure and functionality of a GIS more thoroughly (Chrisman 2002; DeMers 2000). Vine et al. (1997) provide an overview of the use of specific functions in GIS software that could be useful in environmental epidemiologic research. Beyea and Hatch (1999) provide an in-depth discussion of geographic modeling for exposure assessment in environmental epidemiology, as well as an extensive literature review. Briggs and Elliot (1995) provide a review of spatial analysis and mapping in environmental health.