This article adopts a complexity theory framework to review the literature on teachers' professional development practices, the generative systems of these practices, and the impact that learning experiences have on their knowledge and changes in classroom practices. The review brings together multiple strands of literature on teacher professional development, teaching and learning, teacher change, and organizational learning. In doing so, it illustrates that process-product logic has dominated the literature on teacher professional learning and that this has limited explanatory ability. The review demonstrates the ways the elements of three subsystems (the teacher, the school, and the learning activity) interact and combine in different ways and with varying intensities to influence teacher learning. The limitations of studies focusing on specific elements or subsystems are highlighted. The article concludes that to understand teacher learning scholars must adopt methodological practices that focus on explanatory causality and the reciprocal influences of all three subsystems. (Contains 3 notes.)