Starting from the Single European Act of 1986, harmonization of national immigration policies in the course of European integration process yielded varying results for illegal immigrants, asylum seekers and legally resident third country nationals. In this context, this study examines a puzzle: though the rights of legally resident third country nationals have expanded considerably, those of the illegal immigrants and asylum seekers have diminished. It highlights the role played by the securitization process in explaining such puzzle. Thus, this work includes an extensive research on the common European immigration policy with emphasize on the rights of the aforementioned groups as well as the theoretical discussions on securitization. Anyone interested in the common European immigration policy and securitization theory can utilize this work.