Jean d’Aspremont analyses how the Gabčíkovo-Nagymaros Judgment used the International Law Commission’s (ilc) work on international responsibility – and, in turn, the Judgment’s impact on the Articles on Responsibility of States for Internationally Wrongful Acts (arsiwa) – in order to discuss the ‘symbiotic relationship’ between the icj and the ilc in the construction of general rules of international law.