Background: The question of how best to enable sustainable professional development remains a challenging one. The research reported here is situated within a professional learning communities (PLCs) approach as a long-term goal for continuing professional development (CPD) with action research (AR) principles as a means of organisation. Purpose: The aim of the study was to investigate how teachers' CPD can be structured and facilitated to enable sustainable professional development. In this context, 'sustainable' refers to an impact of professional learning beyond the duration of the course in question. Method: An AR-oriented CPD course was used as a case study, with a specific focus on the participants' experiences of how the course work affected their professional practice and the enabling factors contributing to this end. Data were qualitative interviews with the 11 in-service teachers who completed the course, 6 months after its completion. The interviews were analysed in an inductive manner using qualitative content analysis. Findings: The analysis identified eight different categories relating to participants' experiences of how the AR-inspired CPD process affected their practice. Effects were connected to three overarching levels in terms of scope: individual level, school level and level of the wider community. In addition, we determined ten interconnected factors that appeared to influence the scope and quality of the process. Conclusions: The findings highlight the importance of grounding CPD courses in participants' own practice and offer insights into the significance of collaboration and collegial relationships for sustainable professional development. The study reflects how an increase in strategies and structures for collaboration has the potential to transform school cultures into more collaborative ones. It also draws attention to how cognitive and affective aspects are intertwined during developmental processes: teachers suggested that acknowledgement and supportive actions of colleagues and leadership had underpinned development, empowered them and spurred them on as active agents of change.