This chapter introduces the main focus, argument and contribution of the book. We signal that a series of political, social, economic, and environmental crises ask for urgent action and transformative change. Action research offers a mode of knowledge and research that can help generate policy change and sustainability transitions. We highlight that it is increasingly used in the fields of policy analysis and transition research, explain what it has to offer, and demonstrate how it can be engaged in productively. Our core argument is that action research is both critical and relational and that, to productively address the current crises, we need to better engage with this dynamic while (1) negotiating ‘the starting point’ of the research process, (2) enacting ‘multiple roles and relationships’, (3) ‘addressing hegemonic structures, cultures and practices’, and (4) evaluating ‘reflexivity, impact and change’. We provide an overview of how each chapter shares experiences with different approaches to ‘critical and relational’ action research in a variety of geographical and policy contexts.