The European Citizens' Initiative (ECI) is the world's first and only instrument of transnational participatory democracy. It enables one million citizens from at least seven countries to request from the EU Commission the adoption of new proposals on EU legal acts. For this purpose, the organizers have the right to present their requests to the Commission and the European Parliament. The Commission has to examine the requests of the citizens thoroughly and decide whether it intends to take action or not. The ECI has been developed in close cooperation with civil society organizations and members of the European Constitutional Convention on the Future of Europe (2002/03). Since its entry into force in 2012 more than 12 million EU citizens have participated via this new tool of citizen's participation which has led to seven successful ECIs. Given that the ECI has had only limited legal impact one of the open questions is how to improve the ECI effectively so it can reach out to more citizens and help to increase the legitimacy of the EU.