The aim of this article is to propose a theoretical framework describing the internal organization of communities of practice in a dynamic perspective. More precisely, we argue that communities of practice adopt some specific patterns of internal organization where some of their members obtain a leadership status. Leaders contribute to cognitive advance of the community of practice by providing members with a consistent and coherent vision of its objectives. We identify two of their attributes as important for allowing them to fulfil their task: informational mimesis and mediation. Finally, we propose a simulation model describing the emergence of leadership as the outcome of a self-organizing process. We find that leaders correspond to members who are characterized by higher levels of activity in the community.