The need to improve math instruction in the United States has led to a variety of reactionary reform efforts. Often, these reforms require teachers to step away from deeply embedded instructional practices, which are difficult to change. Professional learning communities (PLC) are a common form of professional development in education that have been shown to help teachers make those instructional changes. The purpose of this study was to implement a PLC focused on metacognitive mathematical discourse using a literature-based definition of PLCs. In this practitioner research study, five teachers participated in an eight-week professional learning community focusing on metacognitive mathematical discourse. Qualitative research methods were used to answer the research questions: In what ways do teachers shift to using metacognitive mathematical discourse because of participating in a professional learning community? What experiences in their PLC do teachers indicate as being supportive in terms of shifting their math teaching practices? What attitudes do teachers express about using PLCs to shift their teaching practice? During this study, I facilitated a PLC and documented the process and conducted interviews. Data analysis revealed that when teachers participated in this PLC, they began to shift their instructional practices by first recognizing levels of discourse, then deepening their understanding of metacognition, and finally refining their use of discourse strategies. Teachers changed their practice by exposing their students to more metacognitive discourse without providing explicit instruction. During the PLC, they were able to move towards more explicit strategies by using co-created structures, such as checklists, posters, and exit tickets. However, teachers had limited success engaging struggling math students in metacognitive discourse. Additionally, this study sought to understand the experiences and attitudes of teachers participating in the PLC. Teachers stated that the PLC helped them shift their practice because it was teacher-directed, provided time to collaborate, and encouraged a sense of camaraderie and the normalization of struggle. Although attitudes were more difficult to capture, teachers reported that they found the PLC a supportive workspace. They also reported feeling successful as their students' discourse abilities improved. The results of this study show that PLCs can be effective PD for teachers attempting to shift their math instructional practices. Giving teachers the autonomy to direct their learning as well as create a sustained their learning community can have a positive impact on their instructional practices and their students' use of metacognitive discourse. [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.]