Following participation in a professional development project, teachers from two schools were able to enlarge their community of practice and scale up the intervention school-wide. A bottom-up approach was adopted in the scaling of the intervention. A study of the phenomena in the two schools shows that the impact of the professional development project on student learning enthused other teachers in the schools to join those who had participated in the project, thereby enlarging the communities of practices in the respective schools. The activities of these communities of practice centered on the needs of the teachers, namely, acquisition of new knowledge, use of the knowledge in their classrooms, and feedback on student learning. The activities were facilitated by the resources for teachers produced by the professional development project, and the two main processes adopted by the communities of practice: teachers learning by teaching other teachers; and teachers learning by making their work public and having it discussed and critiqued by their peers.