This chapter examines links between European peaceful borders and the occurrence and proliferation of illicit transnational flows. We refer to the European “internal” borders since the implementation of the Schengen Agreement in 1995, the Southeastern European borders (regarding the Western Balkans/former Yugoslavia) since the end of the Bosnian War in 1995, and the borders between the EU and its Western Balkan neighbors. We assess the softening and the complete opening of the internal borders among the twenty-six Schengen countries in contrast to external border control, as related to the occurrence and proliferation of illicit transnational flows. The two major European concerns in the last two decades are illegal migration and transnational crime, including drug trafficking, human trafficking and smuggling, arms trafficking, and terrorism. Harmonization of internal border controls has abolished the logic of jurisdictional arbitrage. Moreover, peaceful borders that are “hard” between the EU and its Southern and Eastern neighbors remain more controlled, though that it is not necessarily an obstacle for the occurrence of illicit transnational flows.