There has been increased attention to the importance of teachers and teacher quality in developing and sustaining successful education systems for some time now (Barber and Mourshed in How the world's best performing school systems come out on top, McKinsey & Company, New York, 2007; Darling-Hammond et al. in Empowered educators: how high-performing systems shape teaching quality around the world, Jossey Bass, San Francisco, 2017). Germaine to the issue of teacher quality, however, is an acknowledgement of the value of supporting educator's ongoing professional learning (PL). Drawing on findings from a large-scale case study of Ontario, Canada, this paper attempts to shed light on the question of how systems of PL can be developed and scaled? The research was conducted in several phases, including an extensive document analysis, followed by interviews with key respondents across the sector. The paper is framed within the notion of large-scale system change, with a focus on the importance of inter-professional relationships and joint work in establishing shared goals for educational improvement. As we noted in the larger study, a theory of action linked to knowledge of educational change and professional capacity building has informed the mindset, values, and strategies developed and implemented. This paper specifically illustrates Ontario's whole-system's focus on professional learning as part of this wider theory of action.