Social infrastructures are the spaces, facilities, services, systems, and platforms that foster civic interactions and help individuals, families, groups, and communities meet their social needs, maximize their potential for flourishing, and improve community wellbeing, vitality, and resilience. They become participatory when people have agency to contribute their creativity to building collective action projects and collective imagination for the future. This chapter addresses the questions: Why does (participatory) social infrastructure matter, and what are we learning about the potential of the participatory city approach as a critical infrastructure to enable local and societal change? We explore participatory systems initiatives underway in the UK and Canada whose larger vision is to model what societal change can look like when we build regenerative, circular, wellbeing economies from the neighbourhood up. Progress and limitations of current work underway are discussed, as well as intentions for future-building, scaling, and research.