This contribution focuses on the accountability of international governmental organisations as a conceptual problem. To that end it gives a quite extensive summary of the Final Report of the International Law Association on the subject and examines whether accountability can be conceptualise as an actual or potential part of the existing legal system of international organisations. Mainly because of the underlying traditional approach to international (institutional) law, it is asserted that the ILA does not really succeed in this objective. Inspired by a more realistic view of the plurality of modern legal systems – as developed by the institutional legal theory – an alternative legal concept of accountability of international organisations is presented. Based on the fundamental value that legal power entails legal accountability, the legal concept covers all situations of alleged non-compliance by international organisations with a legally valid – but not necessarily also legally binding – rule of international law. For the time being such a concept will be heavily contested in theory and practice. However, for that very reason it can help to stimulate the development of international institutional law as long as the international legal community actually believes in the fundamental value and the divergent views only concern the content of the rules and practices involved in the accountability of international organisations.