The University of Chicago Consortium on Chicago School Research (UChicago CCSR) builds the capacity for school reform by conducting research that identifies what matters for student success and school improvement. This report finds that four years after undergoing dramatic reform efforts such as turnaround, very low-performing elementary schools in Chicago closed the gap in test scores with the system average by almost half in reading and two-thirds in math. The improvements took time to develop; test scores were not significantly better in the first year of reform, but grew larger over time. The study examined five different reform models initiated by the Chicago Public Schools (CPS) in 36 elementary and high schools identified as chronically low performing. The five reform models were: Reconstitution; School Closure and Restart; School Turnaround Specialist Program; Academy for Urban School Leadership; and Office of School Improvement. Each is consistent with one of the four improvement models recommended by the federal government (turnaround, transformation, restart, and school closure). Despite the attention and activity surrounding the models, there is a lack of research on whether or how they work. To begin to address this knowledge gap, CCSR and AIR partnered to examine dramatic interventions in Chicago, an early adopter of such reforms. The report also finds high schools that underwent reform did not show significant improvements in absences or ninth grade on-track-to-graduate rates over matched comparison schools, however recent high school efforts look more promising than earlier ones. Changes in student populations varied across reform models. Schools that underwent these reforms and remained neighborhood schools generally served the same students, and the same types of students, as before intervention. Schools that were closed and replaced with charter or contract schools generally served more advantaged students after intervention. The teacher workforce after intervention across all models was more likely to be white, younger, and less experienced.