The United Kingdom’s 23 June 2016 vote to leave the European Union represents an immense challenge for the Irish peace process. The implicit assumption of continuing British and Irish participation in Europe was woven into the terms of the Belfast Agreement of 1998, which brought an end to three decades of intercommunal violence known as the Troubles. This assumption underpinned guarantees to cross-border participation and equal treatment of the nationalist minority in Northern Ireland, regardless of the minority’s preference for British or Irish citizenship. Even before the referendum on EU membership, the political institutions established by the Belfast Agreement had proven tenuous. Now, the Brexit decision casts further doubt on the settlement’s future.