This chapter discusses the relationship between mass migration, security, and international law. The security implications of migration first depend on what type of migration is at issue and what international legal frameworks accordingly apply to it. The security implications of migration, and the international legal responses to them, also depend on how security is defined. In addition, the transit of migrants may threaten human security, as it may involve loss of life during perilous journeys at sea or during remote land crossings, and exploitative practices such as enslavement or human trafficking. Further, migrants or displaced persons may themselves occasionally present hard or soft security threats. Ultimately, international legal frameworks in relation to migration generally are relatively underdeveloped, including in relation to its security dimensions. Instead, a patchwork of international norms (hard and soft), regimes, and institutions apply to different facets of the migration-security nexus. The chapter focuses on the security dimensions (including terrorism) of international law governing refugees, complementary human rights protection, and due process in the expulsion of aliens (including disclosure of classified information). It identifies gaps and challenges evident in the existing regimes, and charts contemporary developments through soft law initiatives.