This article explored the effects of external agency on the establishment of professional learning communities (PLCs). The research was undertaken in the context of schools that have chosen to adopt the Primary Years Programme (PYP) of the International Baccalaureate (IB) worldwide. The study employed a two-stage qualitative sequential design including data collected from focus groups of administrators, teachers and parents in 122 PYP schools worldwide and case studies of six of these schools provided in-depth understanding of the factors emerging from the focus group data. Findings suggest that external agency can facilitate the development of PLCs that schools, operating in isolation and having to initiate the process without an external framework and oversight, would find difficult to implement regardless of their motivation to do so. The study suggests that in the case of the IBPYP this is done by ensuring schools adopting the programme have a clear vision, mission and ethos linked directly to student outcomes that is transparent, and can be communicated by all members of the school community. The study extends the ongoing research on the development of PLCs and the type of external support most useful in the initial stages of PLC development.