This chapter examines the impact of Brexit on EU-UK cooperation in the context of migration control using the analytical lens of differentiated integration and disintegration. European migration policy has been characterized from the outset by a considerable level of both internal and external differentiated integration. The UK has neither participated in the Schengen Agreement nor has been a full member in the EU’s Area of Freedom, Security and Justice. The UK has refrained from official involvement in Frontex since its creation in 2004. The Agency’s legislative framework, however, has provided the UK with the opportunity to participate in Frontex’s operations and missions aimed at controlling migration flows. Whereas the UK has been engaged in Frontex operations as a means of protecting national territory from irregular migration and preserving national security, the EU has assured British expertise and experience in border and military missions. The empirical analysis of the UK-EU cooperation vis-à-vis Frontex suggests that the partnership in the post-Brexit era is likely to continue in line with the concept of external differentiated disintegration, due to security concerns for the UK and asset gains for the EU.