Participation by the people is a powerful tool which influences the exercise of public authority within all branches of government and may enhance the legitimacy of public authority – but only if it is done correctly. The United States, the European Union and Germany all facilitate forms of citizens’ participation but differ with regard to the actors involved and the degree of influence exerted. Just under fifty years have passed since the participation debate began, yet there is no overarching constitutional theory of participation that encompasses all three branches of government and demonstrates what is meant by “participation done correctly”. This lacuna must be filled in order to further develop participatory mechanisms. This Jean Monnet Working Paper seeks to do so by developing a constitutional theory of imperative participation based on the separation of powers doctrine which may be found in all three legal orders. To do this, the two most important questions about public participation in any given case will be answered: Firstly, who may participate in the exercise of public authority? Secondly, what legal effect does the participation have? In this analysis, participation in the executive’s delegated rule-making procedures – which touches all three branches of government – will serve as the theory’s litmus test.