Professional learning communities can be effective vehicles for teacher learning and instructional improvement, partly because they help change professional culture. However, little is known about "how" these changes occur. We used activity systems analysis to investigate the development of professional learning communities and their teacher leaders (N = 9) based on interviews and observations over 2 years. Activity systems analysis enables researchers to understand change within complex qualitative data sets by finding systemic relations within the activity, specifying contradictions in the system, and explicating outcomes. The resulting activity system illuminates the transformation of a mostly private, autonomous, and egalitarian culture to one of nascent collaboration, reflection, and shared values, as well as teacher leaders' development of agency.