Despite some of the greatest gains in history in terms of high school graduation rates, standardized tests, and college entrance exams, the American education system struggles to effectively educate its neediest and most underserved students, including those in poverty, English language learners, and students of color (DuFour, 2015). Current and widespread strategies used to close the achievement gap in schools are not working (Hanushek, Peterson, Talpey, & Woessmann, 2019). Research indicates multi-tiered systems of support, such as those used in Professional Learning Communities (PLCs), are effective for intervening with students who historically have underperformed in school. Despite multiple research studies that independently validate the effectiveness of the collective strategies used in PLCs, results-based PLCs with a focus on student learning and student outcomes are not widely being implemented (DuFour, 2015). While the effectiveness of implementing PLCs with fidelity has been demonstrated consistently in the research, there does not appear to be research that reveals the commonalities of effective, results-based PLCs in underserved populations. With the knowledge that PLCs are effective in ensuring high levels of learning for all, this study was designed to find the qualities of a results-based PLC in a high-poverty school with a high percentage of traditionally underserved students. The following research question guided this study: What are the qualities of highly effective, results-based PLCs in one high-poverty school with a high underserved student population, and how do those qualities influence the positive academic growth of the students? This case study was conducted in one elementary school in Central California, with a high percentage of low socio-economic students and Hispanic students. Data collected through on-site visitation included document reviews at both the district and site level, observations of grade-level Professional Learning Communities, and semi-structured interviews with a district cabinet member, site leadership, and teacher leaders. The PLC Theoretical Framework (DuFour, 2004) was utilized in this study, which included the four key questions of PLC (What do we want students to learn? How will know if they have learned it? What will we do if have already learned it? What will we do if they have not learned it?) and the three big ideas of PLC (student learning, collaboration, and results).The results of this study revealed three themes describing the most essential qualities driving the highly successful results-based PLCs in the participating school that serves high-poverty, underserved students. First, PLCs must be systemic, district-wide, and implemented with fidelity to ensure results for all students. This includes teacher collaboration, which should only be optional if student learning is optional. This also includes data-driven, "on-purpose" targeted instruction with strong Tier 1 instruction and immediate response to intervention. Second, PLCs must be predicated on a positive and inclusive school culture focused on adult learning. When adults perform, student learning takes place. There must also exist an unequivocal belief within the school culture that all kids are the responsibility of all educators on the school site, and all kids can and must learn. Third, the principal must be a highly effective, trustworthy leader, skilled in both leading PLCs and leading people. Teachers and staff are treated as professionals, devoid of micromanagement; and shared responsibility exists for accountability and student results. High achievement with historically marginalized students is more likely to occur in schools where student achievement is a non-negotiable and a schoolwide collective commitment to student growth and learning exists. The results demonstrate that student achievement for "all" does not need to be an anomaly if a school district is willing to engage in the difficult work of transforming their school system from a system that accepts "some will achieve" to a system that demands "all will achieve.". [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.]