Separation of powers is the time-tested touchstone of the legitimate exercise of power in modern democracies. This collection examines decision-making in the EU’s multilayered and polycentric constitutional structure through this lens.The focus on separation of powers reveals how strong executive powers collaborate in the EU as a single source of public power, which is not sufficiently counterbalanced by parliaments or the judiciary. The collection explores 3 policy fields marked by crisis: the economic and monetary union (EMU), migration, and trade. Drawing on expertise from across these sectors, with a strong conceptual thread linking all the contributions, this important work illustrates how different branches of government co-determine each others’ powers.