The purpose of this study was to examine the professional learning community dimensions of a focus on learning, shared vision, collaborative culture, supportive structures and their impact on student achievement. Additionally, this study explored training in professional learning community principles as a moderating variable and the practice of professional learning community principles as a second dependent variable. A survey was completed by 365 teachers from high, moderate, and low performing high schools on Long Island, New York. Achievement was measured by the three-year average score on the New York State English Language Arts Regents exam typically administered to students in 11th grade. Each school in this study was from the same similar school group as determined by their needs to resources ratio by the New York State Department of Education. The data revealed that teachers from the high performing high school had significantly higher levels of agreement for the dimensions of a professional learning community. Additionally, the professional learning community principles of supportive structures, a focus on learning, collaborative cultures, and a shared vision were all related to student achievement. [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.]