This qualitative research study explored how principal efficacy influences the implementation of Multi-Tiered Systems of Support (MTSS). At the time of this study, the majority of literature focused on efficacy in MTSS focused on the teacher and minimal literature focused on the principal efficacy. Research has explored in-depth the principal as an instructional leader. Research has shown that the principal can have a positive impact on student achievement (Vogel, Weiler, & Armenta 2014; Sheng, Wolff, Kilmer & Yager, 2017). With the principal as the instructional leader in their building, it is important that a principal possess the necessary skills to lead and be a part of the MTSS team. This multiple case study sought to explore principal efficacy of the MTSS process in two suburban districts at the elementary, middle school, and high school levels and how principal efficacy effects the implementation process in their building. Through interviews with principals, this research study specifically examined how training, either in-service or pre-service, influenced principal efficacy and experiences in their implementation of MTSS in their school. Additionally, focus groups with the teachers and individual teacher interviews at each of the buildings examined the implementation of MTSS in their schools. The results of this study provided valuable information to school districts on current levels of principal efficacy in MTSS. With the findings, school districts, Intermediate Units in the Pennsylvania, and principal preparation programs are provided with information on the types of preparation and professional development that principals need in the area of MTSS. [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.]