This qualitative case study looks at school improvement through the on-the-ground experiences of principal supervisors. Beginning with a review of literature on how systems are organized for success, the role of district central offices, and the impact of leadership on improvement, I investigate the work of principal supervisors as they develop principal leadership skills, knowledge and perspectives on implementation of school improvement strategies. Through semi-structured interviews, document review, and a survey, this study finds that principal supervisors: 1) focus on relationship-building with principals as a lever for influencing improvement; 2) focus their time on improving the instructional core (the relationships between task, student, teacher, and content) (Elmore, 2004); and 3) rely on their own experience and expertise to lead principals. Recommendations from this research focus on the training and support of principal supervisors to further enhance effectiveness of this critical role in school improvement and leadership development. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]