On-the-job teacher learning is where most teacher learning occurs. Professional learning communities (PLCs) are one common model for teachers to collaborate and learn from one another. Often, PLCs are organized by grade level to meet regularly for teacher collaboration. When teachers discuss mathematics in PLC meetings, they may be planning instruction, considering assessment results, or reflecting on their students' learning. As the PLC works together, teachers' expertise informs the ideas that are discussed and how the teachers collaborate. In addition, the PLC is one part of the school and district context where teachers focus on mathematics learning. The goal of this study is to understand how teachers' expertise is positioned in the PLC and the larger system of the school and district to inform mathematics teaching and learning. This should help schools and districts understand the features of PLCs that are important for supporting teachers as they collaborate and learn. <br/><br/>This study takes a complex learning systems perspective on teachers' expertise by examining PLCs in the context of the school, district, and state-level activities. The first overarching research question is: How do upper elementary school and middle school teachers experience mathematics-focused PLC meetings that are part of a broader teacher learning system? The second question is: What are the perspectives of various stakeholders within the system on the role of mathematics-focused PLCs in schools? These two questions will be studied via observation data from the PLC meetings, interviews, and surveys. Participants in the study will include elementary and middle school teachers as well as school, district, and state-level leaders to understand their perceptions of the goals and work of the PLCs. Document analysis of artifacts about PLC work in the school and district will be used in conjunction with the interview, survey, and observational data. Epistemic network analysis and social network analysis will be used to understand the connections between individual educators' understandings, the discourse within PLCs, and the collaborative connections amongst PLC members.<br/><br/><br/>The Discovery Research preK-12 program (DRK-12) is an applied research program that seeks to significantly enhance the learning and teaching of science, technology, engineering, and mathematics (STEM) by preK-12 students and teachers. Projects in the DRK-12 program build on fundamental research in STEM education and prior research and development efforts that provide theoretical and empirical justification for funded projects.<br/><br/>This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.