The aim of this chapter is to explore the diversity of action research as it emerge across different approaches and disciplinary fields: from quasi-experimental to participative action research. Variable epistemological and theoretical assumptions systematically lead to the constitution of distinctive forms of action research practices. This diversity is illustrated, with reference to the relevant literature, in terms of variation in: the aims of action research; its conception of knowledge; its conception of change processes; the role of the action researcher and their relations with members.