On the one hand, some federal countries (Belgium, Canada, the United States) have almost entirely devoted water responsibilities to lower levels of government while in other federal states (Australia, Mexico), the central government still plays a strong role (strategic planning, regulation, etc.) On the other hand, though some unitary states still retain significant water responsibilities at central government level with highly centralised water policy making (Chile, Israel, Japan, Korea), most OECD unitary countries (France, Greece, the Netherlands, New Zealand) have de facto devoted responsibilities to lower levels of government. In all cases, the institutional mapping of water policy also relies on environmental, territorial and hydrological considerations. The plurality of mutually dependent actors across ministries and public agencies, between levels of government, and at sub-national level raises significant multi-level governance challenges, hence the need for “diagnosing” capacity and co-ordination challenges likely to hinder integrated water policy.