The process and results of defining neighborhoods may look different depending on the purpose. In this brief, the authors focused on how neighborhoods in Chicago are defined. The initial goal of this work was to answer questions about how access to and enrollment in school-based pre-kindergarten (pre-k) may have varied by neighborhood characteristics. In conducting this work, the authors used a data-driven method for characterizing neighborhoods that leveraged publicly available census data and allowed for consideration of many neighborhood characteristics simultaneously. This method resulted in a parsimonious set of five neighborhood groupings in Chicago that simplified how the relationship of neighborhood characteristics to a host of educational and other outcomes was understood. This brief shares the approach to defining neighborhood groupings and their characteristics and the findings about them. First, the authors review prior neighborhood research and ways of characterizing Chicago community areas and neighborhoods, outlining how the current work builds on these previous efforts. Next the data-driven method for grouping neighborhoods is presented and the neighborhood groupings are described. Finally, the potential uses and implications of the work is discussed. The hope is that others in Chicago (or elsewhere) find this approach to defining neighborhood groupings useful -- both for conducting research studies and providing services or resources.