Our work, a collaboration between the Distributed Leadership for Middle School Mathematics RETA project at Northwestern University and the Math in the Middle Institute Partnership at the University of Nebraska-Lincoln, focuses on professional learning communities (PLC) for mathematics education in middle schools. Specifically, we systematically examine three dimensions of the PLC for middle school mathematics by comparing them to other subjects (e.g., Language Arts). We pay particular attention to the role of math teaching associates in the PLC, trained as part of the Math in the Middle Institute at the University of Nebraska-Lincoln. Further, we analyze relations between key dimensions of the PLC and student learning in mathematics using value-added measures of achievement.