This study was designed to explore the impact of professional development in Professional Learning Communities (PLCs) of secondary mathematics teachers using The DuFour PLC model in one Southeast Texas School District. Descriptive statistics were compiled based on survey data from 19 participants. Internal consistency reliability was analyzed using Cronbach's Alpha. The Wilcoxon signed rank test was utilized to determine if a statistically significant difference between paired observations from the same group of participants occurred in the study. The study's results identified implementations for practice and recommendations for future studies implementing The DuFour PLC model. A noteworthy transformation was the belief that participants felt they could learn from their colleagues and that their colleagues could learn from them because of the work done in the PLC. Data also revealed a very positive shift in the level of acknowledging that the fundamental purpose of The DuFour PLC model is to help all students achieve high levels of learning and, therefore, are willing to examine all practices considering the impact of learning These findings can provide useful information for teachers and school leaders to utilize and implement The DuFour PLC model to improve the foundations of a PLC, teachers' self-efficacy, and student learning. [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.]