What is the federal philosophy inspiring the structure of European law? The federal principle stands for constitutional arrangements that find “unity in diversity”. The two most influential manifestations of the federal principle emerged under the names of “dual” and “cooperative” federalism in the constitutional history of the United States of America. Dual federalism is based on the idea that the federal government and the State governments are co-equals and each is legislating in a separate sphere. Cooperative federalism, on the other hand, stands for the thought that both governments legislate in the same sphere. They are hierarchically arranged and complement each other in solving a social problem. Can the European Union be understood in federal terms? The book’s General Part introduces three constitutional traditions of the federal idea. Following the American tradition, the European Union is defined as a Federation of States as it stands on the “middle ground” between international and national law.