This GDP Working Paper argues that the increasing regulation of immigration detention in European Union law has led to more constitutional protection for immigration detainees. This constitutionalisation has resulted in part from the dynamics between EU law and European human rights law. The complementary relationship between the two legal orders may also contribute to higher standards of protection for immigration detainees. However, denser regulation on the transnational level has at times also resulted in inconsistency, with negative implications for the rights of immigration detainees in Europe. The author concludes that the path towards constitutionalisation is not an inevitable one. It will require constant vigilance and can be encouraged by effective litigation.