This qualitative grounded study investigated professional development, pre-service and in-service that 7 high school principals, 3 assistant principals, and 2 directors in New York State received to build their instructional leadership capacity. The researcher conducted 12 interviews via Zoom video conferencing to ascertain how preparation programs and district professional development for in-service principals and assistant principals prepare leaders to impact student outcomes through instructional leadership. More specifically, the study investigated professional development received on pedagogical strategies, evaluating teacher practice, providing teachers with feedback, and using data to inform instructional decisions in schools. The interviews were used to determine internship experience and the support new leaders received after securing leadership positions. The literature highlighted the importance of instructional leadership to improve student achievement by supporting teachers in their practice. Furthermore, instructional leadership allows leaders to make all decisions with student learning at the core. Literature also revealed the tenets of high-quality preparation programs, which include focusing on instruction and providing potential leaders with mentors and internship experiences that support their development. Effective principals demonstrate the ability to understand and lead instruction in their schools using the latest research and educational trends, highlighting the importance of ongoing professional development while in service. Results revealed that school principals, assistant principals, and content directors lacked a robust preparation program to develop their instructional capacity. Most classes in preparation programs focused on management and theory instead of providing leaders with classes dedicated to providing teachers with feedback, coaching, pedagogy, curriculum supervision, and centering data-driven decision-making. The lack of professional development was prevalent in designing professional development for teachers and disaggregating and using data. Leaders attributed their instructional knowledge to their own backgrounds, self-directed learning, and discussions with their colleagues. Finally, this study provides recommendations for practice and policy and recommendations for further studies that can provide insight to improve preparation programs and district-designed professional development for school leaders to increase student achievement. [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.]