Multi-Tiered System of Supports (MTSS) promotes an equity-based concept of inclusive education as opposed to a place-based concept; thus, the focus shifts from who is included to what is included because the who is all students (McCart & Miller, 2020; Sailor et al., 2018). In this qualitative study, I sought to understand high school teachers' views about the purpose and design of MTSS in general and as related to the extent to which they were prepared, particularly through professional development, to implement Tier 1 MTSS practices. Further, this study identified Tier 1 classroom practices that high school teachers found effective in meeting students' needs in the general education classroom setting. Through semi-structured interviews with 15 high school core content area teachers from schools with MTSS teams, I found that strong MTSS processes are built on what already exists, including effective communication and follow through. All who participated in this study understood the purpose of MTSS and many of them also made a direct connection to special education, explaining that MTSS is not a pathway to classification. MTSS involves teachers monitoring and evaluating student responsiveness to primary, Tier 1 supports to determine whether more intensive Tier 2 or Tier 3 interventions are necessary (Buckman et al., 2021). Tier 1 supports, which are provided to all students in the classroom, were the focus of my study. As a result of this general education effort, teachers can provide strong core instruction and implement best practice for all students. MTSS supports can positively impact high school students' learning outcomes (Bradshaw et al., 2021). All participants shared examples of Tier 1 practices that supported students' needs and credited experience and collegial sharing as the most notable preparation for implementation. This study helped fill the gap in the research on MTSS at the high school level by presenting the results of interviews with high school teachers who shared their views of the purpose and design of MTSS and how they were prepared to implement practices to support the needs of all students in their classroom. Recommendations for future research and recommendations for districts, schools, and MTSS teams are also included. [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.]