In this chapter, we examine the links between peaceful borders among the Southeast Asian countries and the occurrence and proliferation of illicit transnational flows, especially with regard to drug trafficking, human trafficking and smuggling, and arms trafficking. Southeast Asia is the most stable and peaceful among the Asian regions. Regional peace in the Southeast Asian region, especially after 1991, has contributed to its phenomenal economic growth and development, which, in turn, has led to higher levels of integration. At the same time, expanding economic and infrastructure links around region have also facilitated the occurrence of illicit transnational flows, including transnational organized crime and terrorism. The issue of transnational crime addresses the question of national sovereignty. On the one hand, illicit transnational flows pose a threat to the national sovereignty and the territorial integrity of independent states.On the other hand, effective cooperation in combating transnational crime and terrorism requires a political decision by national governments to surrender some parts and pieces of their sovereignty. In this sense, the Southeast Asian countries have traditionally been strong defenders of the sanctity of national sovereignty and nonintervention.