This article introduces the concept of 'co-impact' to characterise the complex and dynamic process of social and economic change generated by participatory action research (PAR). It argues that dominant models of research impact tend to see it as a linear process, based on a donor-recipient model, occurring at the end of a project following the take-up and use of findings. PAR challenges this approach, as impact is embedded in cycles of the action research process; the distinction between researchers, research informants and research users is blurred; and micro process-based impacts, including changes in the thinking and practices of co-researchers, are as significant as findings-based changes in policy and practice. A conceptual framework is developed, based on a three-fold distinction between 'participatory' , 'collaborative' and 'collective' impact. This is applied to a case study action research project, Debt on Teesside, working with low-income households in North-east England. The project is analysed in terms of participatory impact (e.g. developing skills of participating households, mentor-researchers, and university staff ); collaborative impact (e.g. findings-based changes in thinking, policies and practices of advice, community finance and housing agencies, and local authorities resulting from collaborative research); and 'collective impact' , adapted from the field of social interventions, which involves organisations collectively targeting specific actions based on research (e.g. changing policy and practices of lenders and government relating to high-cost loans).