Action Research is now frequently practiced and promoted as a form of continuing professional development for language teachers, with recent studies reporting many benefits for those who engage in Action Research. Less frequently reported, but equally as important for sustainability, are the wider impacts of Action Research engagement on teachers' colleagues, schools and educational sectors. However, to date, there has been no cohesive or holistic review of research on the various forms of development that language teacher Action Research can initiate. This conceptual paper adopts an ecological perspective as a holistic framework for analysing the dimensions of impact of Action Research. An ecological approach studies humans in terms of their relations with their environment, and analyses the affordances and constraints that interact with human development. Accordingly, this paper reviews recent literature on the impact of Action Research in terms of three levels of ecology: micro (individual), meso (school) and macro (wider educational sector) levels. The paper then advances current understandings about why and how Action Research initiates development by analysing the experiences reported, within the studies reviewed, that are unique to teacher Action Research.