GIS have been used at different levels of sophistication in environmental epidemiology studies. These uses range from simply locating the study population by geocoding addresses (assigning mapping coordinates) to using proximity to contaminant source as a surrogate for exposure (Bell et al. 2001; Comba et al. 2003; Langholz et al. 2002; Xiang et al. 2000) to integrating environmental monitoring data into the analysis of the health outcomes (Floret et al. 2003; Gallagher et al. 1998; Reynolds et al. 2002a, 2002b, 2003). However, most of the latter studies have been ecologic in design; relatively few studies have used GIS in estimating environmental levels of a contaminant at the individual level (Nyberg et al. 2000; Reif et al. 2003; Rogers et al. 2000). A number of studies have used GIS to design exposure metrics for use in epidemiologic studies (Bellander et al. 2001; Brody et al. 2002; Cicero-Fernandez et al. 2001; Gunier et al. 2001; Inserra et al. 2002; Kohli et al. 1997; Rull and Ritz 2003; Swartz et al. 2003; Ward et al. 2000). Although yet to be applied in the context of an epidemiologic analysis, several studies have investigated the use of GIS in estimating activity patterns of the study population for potential linkage to environmental data to refine personal exposure estimates (Elgethun et al. 2003; Phillips et al. 2001). Similarly, the use of GIS in spatial statistics for linking exposure and health data in the context of epidemiologic analysis is a growing field of research (Ali et al. 2002; Christakos and Serre 2000; Elliott et al. 2001). This article is a discussion of the fundamentals of the scientific disciplines required to use GIS in exposure assessment for epidemiologic studies and explores how a GIS can be used to accomplish several steps in the exposure assessment process (those shaded blue in Figure 2). Specifically these steps are a) defining the study population, b) identifying source and potential routes of exposure, c) estimating environmental levels of target contaminants, and d ) estimating personal exposures.