The onset of the first decade of the 21st century has seen many district change initiatives within the New York City Department of Education. Several of these district initiatives have been instructionally-based and thus have made the role of high school supervisory assistant principals integral in their successful implementation. The purpose of this study was to examine high school supervisory assistant principals' leadership roles in implementing district change initiatives such as the New York State Annual Professional Performance Review (APPR) system, which evaluates teacher effectiveness. Data were collected via in-person interviews that garnered qualitative data from New York City Department of Education high school supervisory assistant principals. Themes that emerged from this grounded-theory study were the importance of assistant principal leadership in the implementation of the APPR, building internal instructional capacity for the implementation of the Common Core State Standards, teacher evaluation and observation as a vehicle for change, assistant principals' impact upon student achievement through their instructional leadership in supporting teachers using the APPR, assistant principals' role in supporting student achievement within and outside of the classroom, the importance of assistant principals' creativity and flexibility, and lastly, distributive leadership and collaboration. The practical implications of this study are to guide future research into developing coherent standards for high school supervisory assistant principals' roles in their school building setting and also in framing high-stakes accountability measures for assistant principals under the APPR system within the New York City Department of Education. [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://www.proquest.com/en-US/products/dissertations/individuals.shtml.]