Abstract: This chapter approaches federalism from an overtly theoretical perspective, seeking to explore the nature of federalism as an idea rather than as a particular institutional model. It argues that federalism is essentially a constitutional idea, and therefore must be addressed through the specific disciplinary prism of constitutional theory. Existing federal theory has tended to be bound up with, and informed by, the surrounding ideological environment within which it has been shaped, in particular liberal democracy. A key contention of the chapter is that the concept of federalism as a constitutional idea requires to be retrieved from the thick normative baggage with which it has been burdened. It is also argued that the federal idea is one that has itself mutated through the evolution of federal practice over time and from place to place. The task of extricating the constitutional idea of federalism is one which must ground itself not only in theories of constitutional government but in the comparative study of the lived reality of that constitutional idea from time to time and from place to place.