Examining the legal effects of EU concluded treaties, this book provides an analysis of this increasingly important and rapidly growing area of EU law. The EU has concluded more than 1,000 treaties including recently its first human rights treaty (the UN Rights of Persons with Disability Convention). These agreements are regularly invoked in litigation in the Courts of the member states and before the EU courts in Luxembourg but their ramifications for the EU legal order and that of the member states remains underexplored. Through analysis of over 300 cases, the book finds evidence of a twin-track approach whereby the Court of Justice of the European Union (CJEU) adopts a maximalist approach to Treaty enforcement, where EU agreements are invoked in challenges to member state level action whilst largely insulating EU action from meaningful review vis-à-vis agreements. The book also reveals novel findings regarding the use of EU agreements in EU level litigation including: the types and which specific EU agreements (including the types of provisions) have arisen in litigation; the nature of the proceedings (preliminary rulings or direct actions) and the number of occasions in which they have been addressed in challenges to member state or EU action and the outcomes; who has been litigating (individuals, institutions, or member states) and which domestic courts have been referring questions to the CJEU. The significance of the judicial developments in this area are situated within the context of the domestic constitutional ramifications for member state legal orders thus revealing a neglected dimension in the constitutionalization debates, which traditionally emphasized the ramifications of internal EU law for the domestic constitutional order without expressly accommodating the constitutional significance of this external category of EU law nor the different challenges that this poses domestically.