In 1996, the Chicago Public Schools (CPS) began implementing a new school accountability policy designed to improve student performance by providing a combination of consequences and support to low-performing schools. The center point of the accountability system, the Chicago school probation policy, designates schools as being "on probation" if fewer than 15% (later raised to 20%) of their students score at grade-level norms on the Iowa Test of Basic Skills in reading. When placed on probation, schools face the consequences of decreased autonomy and the threat of more severe sanctions. At the same time, probation schools receive direct assistance from several different sources through the policy's external support system. The purpose of the support is to assist schools in strengthening their internal operations, raising expectations for students, and improving instruction so as to foster increased student achievement. This report is based on a two-year study of the design and implementation of the school probation policy in Chicago's elementary schools. The school accountability system in Chicago has undergone changes since the end of this study as a result of new district leadership. The system now includes the use of additional assessment data and subject area tests, emphasis on progress and growth, and a focus on all schools. In spite of these changes, the assistance provided by probation managers and external partners has not changed. Therefore, the lessons learned from this study should still be relevant not only to CPS but also to other jurisdictions instituting similar policies.