Citizens are critical actors in sustainable transitions. Via their (consumer) behaviours, adoption of sustainable practices and technologies, grassroots initiatives, local actions, citizen science, protest, and involvement in participative decision making, they entail considerable influence. Recently, there is increasing attention to collective citizen action, in particular in the domain of the sustainable energy transition. Community energy collectives, often in the form of renewable energy cooperatives, are a manifestation of civil society in the energy domain. They engage in agency to achieve sustainable energy goals, lower carbon emissions, and foster social innovation. They engage with local communities, target group persuasion and embark on complex organizational processes to develop projects. To tackle institutional inertia, renewable energy cooperatives take on experimentation and lobby for institutional change. This chapter presents an overview on how renewable energy cooperatives use their agency to achieve sustainable transition goals. It ends with suggestions for future research.