Innumerable attempts have been made to explore the theoretical nature of federalism. Due to the long history, worldwide existence and interdisciplinary character of federalism, a plethora of literature has been written on the topic. Yet, these endeavours have not even resulted in a clear and commonly used definition of the term. Surely, it is one of the great dilemmas of this field of research that despite so much discussion, there is no settled common denominator of ‘federalism'. Whereas practical studies and exchange of experience between the various federal systems offer a more conventional research arena, comparative theoretical approaches are much more seldom. This is not the least because of the tremendous semantic challenges of a comparative theoretical approach. At first glance, it is sometimes difficult to understand the terminology of federalism, the meaning of which differs according to the perspectives of constitutional law, political science or economics. Even more difficulty arises when the substance of federal theories is discussed. Again, differences between theories may be due to different academic approaches, particularly between understanding federalism as an overall principle or as a more concrete concept of a federal state and, in particular, whether the constituent units of a federal state are states, and, if states, whether they are sovereign.