The overall objective of the project is to generate systematic knowledge about the interaction of international treaty systems and EU environmental instruments in both its horizontal di-mension (i.e. between instruments at the EU-level as well as between international treaty sys-tems) and its vertical dimension (between international treaty systems and EU-instru- ments), and to derive suitable policy recommendations. To this end, the project will, first, develop a theoretical concept that categorises phenomena of institutional interaction. Second, it will elaborate comprehensive inventories of cases of interaction for four international environ-mental treaty systems, the world trading system (GATT/WTO) and High-Seas fisheries man-agement, as well as for six EU environmental instruments. Third, selected cases of institutional interaction are to be explored in depth within each case study in order to analyse their consequences and available policy responses. Fourth, a comparative evaluation will produce generalised knowledge on institutional interactions and their impact on the effectiveness of international treaty systems and EU environmental instruments. On this basis, policy recom-mendations are to be developed on how to avoid and mitigate conflicts or enhance possible synergies arising from such institutional interaction.