The law of international organizations is undergoing profound changes. This has been caused in large part by the increasingly important role that international organizations have played in exercising powers conferred on them by national governments. This phenomenon has led to concerted attempts by states, international courts and tribunals, and domestic courts to seek to ensure accountability for these exercises of power by imposing corresponding limits on international organizations.This chapter focuses, first, on the development of international law relating to the legal personality of international organizations, including in this context a brief consideration of the issue of immunity; and then, second, turns to discuss the relationship between states and international organizations and the implications of this relationship for the responsibility of states and in some cases the responsibility of international organizations.</p