How far does the ongoing process of constitutionalization in the European Union extend beyond the circle of the member states and their citizens to include the codification of individual rights relating to third country nationals? In the light of a restrictive migration acquis, the reasons for the gradual codification of aliens' rights and the conditions under which this (contested) codification takes place are not well understood. Examining the main constitutional and legislative steps in the area of EU asylum and immigration policy, I will argue that successful constitutionalization depends mainly on the effective linkage of rights with well-established norms embedded in the European integration process.