This article considers the constitutional requirements and implications of Article 50 TEU for the EU. It argues that it is essential to read Article 50 in light of the features of the Treaty of which it forms part together with its drafting context, that of the Convention on the Future of Europe, as well as the substantive protections of EU constitutional law. The article demonstrates that important constitutional constraints are in place in EU law, which can affect the most significant debates in the withdrawal process, namely: the manner in which notification to withdraw from the Union is given; the revocability of a decision to withdraw; and the legal basis and content of the withdrawal agreement. Most importantly, a reading of Article 50 informed by key constitutional features of the EU legal order stipulates clear duties for the EU to respect the UK’s constitutional requirements and to protect, in any eventual agreement, acquired rights for EU citizens in the UK and UK citizens in the EU, by emphasizing the illegality of a non-compliant withdrawal agreement from the EU perspective.