State and federal governments have been increasing school accountability requirements and relying on school administrators to promote improvement. Anecdotal evidence suggests that schools and districts are finding it difficult to attract and retain qualified people to serve as administrators because there is a shortage of such people, but recent empirically based analyses show there is no shortage. This report explores this issue further, adding to the current understanding of school administrators’ careers through an in-depth analysis of administrative data from the state of Illinois. A comprehensive, descriptive overview of school administrators is presented, and the individual is modeled based on school characteristics associated with various career transitions. The authors describe in detail what state-level administrative data can reveal about the careers of school administrators, what the data cannot reveal, and how further research and data collection might be directed so that systematic administrative data can be used to better understand the relationship between administrative career paths and student learning outcomes. This report is one of a group of companion reports; the others cover similar analyses for New York and North Carolina.