The purpose of this mixed methods collective case study was to explore the utilization of assistant principals as instructional leaders to identify and explain factors that influence instructional leadership effectiveness of assistant principals, and the potential effect on school performance in northeastern Pennsylvania secondary schools. The desired outcome of the research was to establish a relationship between the instructional leadership of the assistant principal role and school performance. The research provided justification for exploring the job descriptions, preparation programs, and evaluation processes of the assistant principalship to better align them with instructional leadership objectives, as opposed to managerial and disciplinary functions. "Despite their importance as school leaders and future principals, the role of the assistant principal is not well understood" (Moyer & Goldring, 2023, p. 41). The three research questions that were answered were: (1) What is the relationship between the utilization of APs and the academic performance of high schools in northeastern Pennsylvania? (2) What are assistant principals' perceptions of their roles as instructional leaders in northeastern Pennsylvania high schools? (3) How are assistant principals' instructional leadership activities measured and evaluated in northeastern Pennsylvania high schools? Data was collected using one-to-one semi-structured interviews, instructional leadership surveys, activity logs, and artifacts. Qualitative analysis was conducted using In Vivo and Concept Coding methodologies, while quantitative analysis included Pearson Correlation techniques. [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.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]