This mixed methodology phenomenological case study, which included an explanatory sequential research design, explored the behaviors principals employ when promoting fidelity to universal Multi-Tiered Systems of Support for Behavior (MTSS-B), otherwise known as Positive Behavior Interventions and Supports (PBIS). The target population consisted of 62 principals and 90 staff members from Pennsylvania schools who met universal fidelity during school years 2019-20 and 2020-21. The school version of the Implementation Leadership Scale (S-ILS) was used to measure the social validity of staff and principal extent ratings of principal behaviors in support of evidence-based Tier 1 PBIS practices. Independent t-tests measuring the significance between staff and principal mean scale scores indicated principals rated themselves significantly higher on the availability subscale than staff. Fourteen principals who participated in the S-ILS survey were invited to participate in focus groups using critical incident technique to further delve into principal behaviors promoting MTSS-B/PBIS and mitigating implementation challenges. Five themes emerged identifying principal behaviors promoting fidelity: (a) focusing on relationships, (b) preparing staff to implement, (c) supporting the leadership team, (d) promoting the PBIS process, and (e) sustaining fidelity. Challenges to implementation included (a) challenges with the leadership team, (b) stakeholder resistance, (c) resource scarcity, and (d) consequences of COVID-19. When faced with challenges, principals described critical incidents where they were supportive, proactive, and communicative. Implications for theory and practice, study limitations, and recommendations for future research are also provided. [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.]