At City Academies Neighborhood Middle School, there is a gap in student achievement. Over the course of two years, our lowest performing students did not grow at the rate of their peers. This problem has several consequences, including over classification of students with disabilities and a widening achievement gap. The purpose of my research was to determine if increasing teacher knowledge and implementation of Tier I differentiation positively impacts student learning outcomes. This research focused on two English Language Arts teachers and 71 eighth-grade students. The data for this study was collected through professional learning communities, student assessments, and teacher interviews. The findings in this project note that increased professional learning, with a focus on Tier I differentiation and student data analysis, positively affects student learning. These findings indicate the need for a change in the structure of professional learning communities and increased focus on Tier I differentiation in teacher professional development. [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://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]