In the last few years in international law, more and more regulation comes in the form of post treaty rules. This book offers the first systematic examination of the rules, which are made up of a combination of decisions, regulations and recommendations, to assess their efficiency. The study shows that the authority of these rules is in question as, in practice, member states retain almost complete discretion in their exercise. This conclusion gives rise to the key question; to what extent does this ambiguous authority affect respect/adherence to procedural principles in the practice of the application? This in turn leads to more far-reaching questions of legal certainty, non-arbitrariness and the duty to state reasons. In assessing this question of legitimacy, the study shines a light on this crucial but neglected question in international law scholarship and forms a starting point for improvements and reform.