The UK‘s 2016 Referendum on continued EU membership may well become a defining moment in the constitutional law and politics of the UK. Undoubtedly, untangling and re-establishing a relationship with the EU and the wider world will have legal, economic and social effects within the UK, as well as in remaining member states. But Brexit is as much about the British Constitution as it is about economics and relations with continental Europe. Brexit challenges a constitution that is already ‘unsettled‘ (Walker) and provokes a range of constitutional problems, which include: confusion over constitutional requirements for commencing the withdrawal process; the respective roles of the UK Parliament and Government in Britain‘s EU withdrawal; the position of the devolved nations and the future of the territorial state; and the domestic legislative changes necessary The paper will locate Brexit in the broader context of historically significant British acts of union or disunion.