Geographic information systems (GIS) are increasingly being used in public health and medicine. Advances in computer technology, the encouragement of its use by the federal government, and the wide availability of academic and commercial courses on GIS are responsible for its growth. Some view GIS as only a tool for spatial research and policy analysis, while others believe it is part of a larger emerging new science including geography, cartography, geodesy, and remote sensing. The specific advantages and problems of GIS are discussed. The greatest potential of GIS is its ability to clearly show the results of complex analyses through maps. Problems in using GIS include its costs, the need to adequately train staff, the use of appropriate spatial units, and the risk it poses to violating patient confidentiality. Lastly, the fourteen articles in this special issue devoted to GIS are introduced and briefly discussed.