One of the purposes of this book is to introduce community health groups to the potential of GIS, a technology that can help in understanding the spatial landscape of prenatal risk. Therefore, one of the first steps is to provide a brief overview of GIS. This introduction will be split over the next two sections, with this chapter focused on data issues associated with using a GIS, while the next presents an introduction to the functions of a GIS that make it so powerful: the ability to analyze and visualize spatial data. These two chapters are not meant for experienced GIS users (though even for these a few points and references from the non-geographic literature may prove to be useful). It is also not meant to be a comprehensive introduction to the science; there are several other excellent texts serving that need. These next two chapters are meant to give a basic understanding, and inform enough to encourage the adoption of a GIS approach. Most people reading this book will probably be using a vector GIS. There are, however, two basic GIS formats, raster being the other type. Raster GIS is best suited for surface or complete coverage data (for example, vegetation cover) because the spatial surface is transformed into a grid, with each cell or pixel containing a relevant geographic attribute (such as 1 = water, 2 = forest, etc.). These pixels are fixed in space, allowing multiple layers at a single location to be compared and analyzed. Vector GIS, which is a more useful GIS format for the type of investigation likely to be performed by a health unit, contains points, lines, and areas. Unlike in a raster GIS, each spatial object has its own geography.