This paper explores professional learning as teachers engage in action research to improve their practices. Despite many contributions on professional learning communities and their effects on school improvement, there is less research on how they are developed and how teachers learn collectively. Using a social theory of learning, three dimensions of communities of practice are used to analyse and describe the qualities of learning groups, where teachers participate by conducting action research. Findings suggest that when teachers engage in action research, they develop a shared repertoire related to the local needs, as well as to the field and traditions of action research. In turn, the repertoire facilitates collaboration among teachers. However, different understandings of the project affect mutual engagement in the learning groups. This indicates that deficiencies in alignment between the local practice and the practice of action research, affect the group qualities and may hinder the development of a PLC. Hence, this paper argues for the importance of making explicit the learning goals for both the local schools' improvement work and the new way of working, which is the refinement of the practice. The implications for practice in schools entail making practice visible, including each school's learning history.