This chapter maps the challenges that Brexit might pose to the peacebuilding processes in Northern Ireland and Cyprus. It argues that the decision of the UK to leave the single market and the customs union threatens cross-border cooperation and as such creates tensions to the political and economic life of those two segmented societies. At the same time, the UK’s withdrawal from the EU questions the fragile balance that has been achieved on the question of sovereignty of those regions through the Good Friday Agreement and the 1960 Agreements related to Cyprus. The relevant Protocols of the UK Withdrawal Agreement try to address this issue by accepting some differentiated arrangements for the two regions post-Brexit and thus creating some continuity to their current EU legal status. However, while the idea of a differentiated Brexit has been accepted for the Sovereign Base Areas in Cyprus, it has been vehemently rejected for Northern Ireland.