The purpose of the study was to assess the impact that professional learning communities (PLCs) have on student achievement. Through this study, the statewide performance results of a school district that implemented PLCs were examined. This study of an Illinois school district's statewide assessment results served as a foundation for further analysis of the academic achievement of students who are taught in schools that implement PLCs. The method that was utilized in this study is the quantitative research design. The research demonstrates an ex post facto study under the causal-comparative design. The causal-comparative design explained the difference that occurred within a group after the implementation of PLCs. Data were collected from the Illinois Standards Achievement Test (ISAT). This instrument was used in this study to determine the effect on student achievement. The ISAT was designed to measure how well students attain, absorb, and achieve the knowledge and skills set forth in a specific unit of instruction (Illinois State Board of Education, 2008). The second instrument used in this study was Hord's (2004) School Professional Staff as Learning Community Questionnaire. The purpose of this instrument was to detect whether schools are implementing the five dimensions of PLCs as outlined through the review of literature. The implications of this study revealed that PLCs do impact student achievement. Administrators are the main leaders in the building and have a responsibility to develop support conditions and lead the school on the path of creating a shared vision and values. Administrative support is purposeful in the development and maintenance of PLCs, according to the research. This study illustrates that PLCs can impact student achievement when implemented within the guidelines outlined by the experts, such as Hord (2004) and DuFour (2008). The goal is ultimately to increase student achievement, and it has been proven that this type of learning community has the capacity to make that happen. [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.]