© 2017 Elsevier B.V. Understanding the diversifying role of civil society in Europe's sustainability pathway is a valid proposition both scientifically and socially. Civil society organisations already play a significant role in the reality of cities, what remains to be explored is the question: what is the role of civil society in the future sustainability of European cities? We first examine the novelty of new forms of civil society organization based on a thorough review of recent case studies of civil society initiatives for sustainable transitions across a diversity of European projects and an extensive literature review. We conceptualize a series of roles that civil society plays and the tensions they entail. We argue that, civil society initiatives can pioneer new social relations and practices therefore be an integral part of urban transformations and can fill the void left by a retreating welfare state, thereby safeguarding and servicing social needs but also backing up such a rolling back of the welfare state. It can act as a hidden innovator—contributing to sustainability but remaining disconnected from the wider society. Assuming each of these roles can have unintended effects, such as being proliferated by political agendas, which endanger its role and social mission, and can be peeled off to serve political agendas resulting in its disempowerment and over-exposure. We conclude with a series of implications for future research on the roles of civil society in urban sustainability transitions.