ACTION will transform the way we do citizen science (CS) today: from a mostly scientist-led process to a more participatory, inclusive, citizen-led one, which acknowledges the diversity of the CS landscape and of the challenges CS teams have to meet as their project evolves. We have partnered with 5 European CS initiatives tackling major forms of pollution, which pose substantial threats to human health and to the environment, and contributing to Sustainable Development Goals. These pilots will be the starting point for a ‘citizen science accelerator’, which will be expanded through an open call. By considering the needs of multiple stakeholders throughout the lifecycle of CS, we will create methodologies, tools and guidelines to truly democratise the scientific process , allowing anyone to design and realise a CS project from the early stages of ideation to validating and publishing the results. Our research will account for the multitude of manifestations of CS, addressing everything from from small-scale, localised social issues to international research agendas. All ACTION’s outputs - infrastructure, the citizen science platform and toolkit - will be made openly available for online and offline use. They will use accessible formats and interfaces, which appeal to audiences with diverse motivations and backgrounds and provide detailed examples, workflows, and advice tailored for a range of activities, going beyond data collection and analysis. Our digital infrastructure will help citizen scientists use existing specialised platforms and publish results according to RRI principles, including open science. Our toolkit will tackle common difficulties around methodological choices, quality, incentives, community building and sustainability. In addition, the 35 pilots hosted by the accelerator will result in case studies that will demonstrate the impacts of CS at social, economic, environmental and policy level.