A prominent feature of education discourse relating to teachers' practice has been the call for increased emphasis on professional development (PD). This paper is part of a wider project which explored the impact of a collaborative PD initiative on teachers' teaching and learning in five urban disadvantaged schools in Ireland. It focused on the impact of PD and critically on sustainability from which emerged important issues of leadership. This article focuses on the specific contribution that leadership made to the sustainability of the practices in the schools. It aims to identify three key features of how principals contribute to sustaining PD practices.