Human migration can make it more difficult to detect geographic differences in disease risk because of the spatial diffusion of people originally exposed in a given geographic area. There are also situations where migration can facilitate the detection of disease attributable to environmental hazards. This paper assesses the effects that migration has on the ability to detect regional variability in disease risk. Several characteristics of migration are discussed, including some that are not widely known. Because of regional variations in mobility rates and other characteristics of the migration process, there is substantial regional variation in the ability to detect spatial variation in risk.