We present results from a five-year study of secondary science and mathematics teacher professional development involving collaborative Professional Learning Communities (PLCs). Over 200 hours of classroom and PLC video were selected for in-depth analysis using techniques of open, axial, and selective coding. This analysis resulted in the emergence of three central categories of Process Behaviors characterizing the degree to which teachers inquired into topics of teaching and learning in a scientific manner and three categories of Dispositional Behaviors characterizing their approach to discourse. A central construct of Decentering emerged for following facilitator moves to manage the discourse, and we identified four categories of Teacher Beliefs that influenced the ability or willingness to change classroom practices. We also discuss relationships among these categories that emerged from the research and implications for design and implementation of teacher professional development.