The EU Water Framework Directive introduces the principle of integrated river basin management, incorporating both the idea of spatial fit between ecosystems and social systems and a requirement to integrate water management across scales and sectors. In designing their implementation setups, member states must therefore address both the roles of different institutional actors and the interplay among institutions. In this paper, we will explore strengths and weaknesses of different institutional arrangements for integrated water management through a comparative analysis of River Basin Management Planning processes in six countries around the Baltic Sea. We use theories on multi-level governance, regime interplay and institutional effectiveness. We find that, in most cases, central governments have played a dominant role in the formulation of river basin management plans, while local influence has been somewhat limited. The tight procedural deadlines of the di-rective appear to have pushed for more centralisation than originally intended by the countries. But the analysis also shows that interplay mechanisms such as norms, ideas and incentives do promote effective institutional interplay and may serve to overcome coordination problems of im-plementation structures. Moreover, it is expected that institutional interplay will improve over time resulting in more integrated management.