Terrorism is not new to Southeast Asia. For much of the Cold War, the activities of a variety of domestic ethnonationalist and religious militant groups posed a significant challenge to the region’s internal stability. Since the 1990s, however, the residual challenge posed by substate militant extremism has risen in reaction to both the force of modernization pursued by many Southeast Asian governments and the political influence of radical Islam. Building on prior RAND research analyzing the underlying motives, drivers, and capabilities of the principal extremist groups that have resorted to terrorist violence in the Philippines, southern Thailand, and Indonesia, this study examined the historical roots of militancy in these countries to provide context for assessing the degree to which local agendas are either being subsumed within a broader ideological framework or shaped by other extremist movements. Moving beyond simple terrorism analysis, this research also examined national and international government responses to militant movements in the region, including counterterrorist initiatives, military and policing strategies, hearts-and-minds campaigns, and funding and support from international organizations and governments (including the United States). Finally, the study broke new ground in assessing Cambodia as a potential future terrorist operational and logistical hub in Southeast Asia.