In this paper, I explore the experiences of secondary teachers in four London schools [UK] who participated in Teacher Learning Communities, defined as meetings in which professional learning was supported as they learned about Assessment for Learning (AfL). The claim for these communities is that they lead to sustained improvements in teaching and learning, where the following design principles are adhered to: where leaders respect and value a need that has been identified by participants as of importance to themselves; they are school-based and integral to school operations; there is teacher collaboration; and there is input from within and beyond the school to support teachers' theoretical as well as practical learning. The findings from this research project suggest that Teacher Learning Communities' benefits were compromised specifically: where they were imposed on teachers; where they were not accommodated sufficiently within other school commitments; where leaders were too directive; where meeting formats were adhered to inflexibly; and where practice was emphasized at the expense of theories. My conclusion is that both AfL and Teacher Learning Communities rely for their success on sustained critical reflection among their participants, which can be inhibited where the above limitations apply.