International Political Sociology (IPS) literature mainly focuses on the actions of the European Union (EU) in their analyses of externalization in the Mediterranean, examining how border security policies and practices of the EU actors constitute insecurities for multiple referents. The thesis shows that most IPS studies either overlook or underemphasize how non-EU actors also play a significant role in creating these insecurities. It argues that non-EU actors have agency in the constitution of the EU’s external border in/security policies and practices. The thesis also argues that, with its meta-theoretical and theoretical commitments, IPS is a suitable approach to locating multiple actors’ agency in the making of in/security. Through focusing on the case of Turkey, the dissertation concludes that border in/security cooperation between EU and non-EU states of the Mediterranean may be unbalanced in nature, but non-EU actors still have agency in the constitution of in/security.