Recent UN activities have raised legitimacy concerns for which the UN itself has suggested solutions based on the rule of law. But neither the UN Declaration on the rule of law nor existing legal theory has provided a precise definition or a more detailed explanation of the rule of law. A study of the rule of law applying to the UN has to respect the specific features of the international legal order, i.e.: its highly political context; the lack of an overarching constitutional framework and of the idea of the separation of powers; and the corresponding need for flexibility as a consequence of the involvement of the Member States on which the UN depends. With these features in mind, the new approach of ‘modified functionalism’ is developed in this chapter to explain the legal basis and the binding nature of the rule of law as it applies to the UN.