This chapter focuses on the role of Association of Southeast Asian Nations (ASEAN) in the fight against ISIL. It summarizes the responses and efforts of ASEAN member nations to address the security threats. The chapter identifies several challenges the member nations of ASEAN must address in an attempt to develop a cohesive and effective response to ISIL and the overall threat of terrorism in the region. ASEAN members participated in capacity-building activities to strengthen regional counter-terrorism efforts. The governments of Singapore, Malaysia, and Indonesia all have Internal Security Acts (ISAs), or pending replacements, used to detain and arrest individuals suspected of terrorist-related activities. Diverse experiences with terrorism and varying levels of domestic political stability influenced individual state stances toward regional counter-terrorism cooperation. Domestic constraints and in particular government legitimacy continue to play a role in progress toward organizational cohesiveness and unity.