In analyzing European institutional structures, the multi-level governance literature speaks of mutual interdependence, of cooperative and coordinated processes between different levels of authority. The emergence of these processes is sometimes associated with the disappearance of hierarchical relationships organized across vertical channels of communication. The type of hierarchy, which is declared 'eroded', is not further specified. In the first part of the article a definition of hierarchy is given and an attempt is then made to develop a diversified concept of hierarchy consisting of two types: unilateral and asymmetrically interdependent. The second part applies this modified concept of hierarchy to relationship structures in those federations which are part of European multi-level polity, and examines the types of hierarchy that exist between federal and federated levels when the latter act within the European framework. The external policies of German and Belgian federated units are analyzed in a comparative perspective.