Action research (AR) is becoming increasingly popular in ELT contexts as a means of continuous professional development. The positive impacts of AR on language teacher development are well documented, but the important question of how those impacts can be sustained over time is virtually unexplored. Drawing on findings from a study of teachers in Australia, we address the question of the sustainability of the impact of AR. Data from a survey and interviews show that, between one and four years after completing an AR programme, the teachers felt more confident, connected to their students, research-engaged, and recognized by colleagues and managers. We argue that a balance of top-down institutional support and individual teacher motivation is essential in ensuring sustainability of the impact over time. Finally, we suggest how the benefits of AR can be sustained for teachers doing AR and their colleagues.