Three years after the introduction of the European Citizens' Initiative, the present volume offers a critical, but nonetheless cautiously optimistic perspective on the opportunities and constraints of this novel tool of transnational participatory democracy. Bringing together a wide range of ECI researchers, the volume combines theoretical discussions on the nature and contribution of the ECI with empirical perspectives on the experiences of the first initiatives launched since April 2012. The contributions show that the ECI is difficult to place as an instrument of participatory democracy, owing to considerable ambiguity as regards its merely indirect agenda-setting function. More importantly, the ECI has so far only in part been able to fulfill the high expectations held by academics, activists and ECI organizers alike. The conclusions of the volume are, therefore, somewhat ambivalent: while the ECI clearly has significant democratizing potential, the experience of the first three years also points to a considerable need for reform.