The United Kingdom and Ireland provide a unique, live experiment in state transformation and constitutional change. Since 1999, the UK has been in transition from a unitary state into a complex, plurinational union in an open-ended process, which leaves numerous unresolved issues. This has occurred during UK membership of the European Union, which has provided a framework and external support system for internal devolution, while facilitating a new relationship with Ireland. The British and European unions have, for the most part, been compatible and mutually sustaining. Brexit, whatever form it finally takes, destabilizes what was already a fragile settlement and forces the pace of constitutional change. The panel will consider issues raised by the conjunction of Brexit and constitutional change within ‘these islands’.