One of the most signifi cant developments in the evolution of the international legal order over the last hundred years or so has been the emergence of international organizations (IOs) as autonomous legal actors. Whilst this autonomy is expressed formally in the recognition of an organization’s international legal personality, its ‘separate will’ manifests more concretely in its ability to exercise legal powers, in turn depending on the internal ‘constitutional’ dynamics at play in the organization, particularly between institution and member-states. Examining this relationship in relatively centralized institutions like the United Nations (UN), however, reveals complex layers of autonomy. Whilst this layering undoubtedly confi rms that these institutions can possess a separate will, which is clearly more than the sum of their separate parts, these complex internal dynamics make it somewhat diffi cult to develop a coherent system of accountability or legal responsibility to control their activities: such responsibility being perhaps the most obvious corollary of legal autonomy.