Southeast Asia is home to more than 625 million people and around 15% of the world’s Muslim population. The region has faced the threat of terrorism for decades, but threats in Southeast Asia have never been considered as great as threats in some other regions. However, the rise of the Islamic State poses new, heightened challenges for Southeast Asian governments and for U.S. policy towards the region. Southeast Asia has numerous dynamic economies and three Muslim-majority states, including the world’s largest Muslim-majority nation, Indonesia. Although the mainstream of Islamic practice across the region is comparatively tolerant of other religions, Southeast Asia is also home to several longstanding and sometimes violent separatist movements and pockets of Islamist radicalism, which have led to instances of violence over the past 30 years. These were particularly acute during the 2000s, when several attacks in Indonesia killed hundreds of Indonesians and dozens of Westerners. The threat seemingly eased in the late 2000s-early 2010s, with the success of some Southeast Asian governments’ efforts to combat violent militancy and degrade some of the region’s foremost terrorist groups. Several Southeast Asian governments, including Indonesia, Malaysia, and Singapore, have intensified counterterror efforts since 2014, outlawing calls for support of the Islamic State and strengthening policing and border-control efforts. Yet the challenges that governments in the region face were exemplified in January 2016 by a violent attack in Jakarta, Indonesia, that killed eight people, including four civilians. There are several factors that characterize the terrorism threat in Southeast Asia. The region’s largest Muslim-majority nations, Indonesia and Malaysia, have long been known for moderate forms of Islam and the protection of religious diversity; policies that have widespread popular support but which raise resentments among small numbers of conservative actors. In other Southeast Asian countries with substantial Muslim populations, including the Philippines and Thailand, simmering resentments in Muslim-majority regions have been fed by perceived cultural and economic repression, leading to separatist movements that have posed threats to domestic groups and, in the case of the Philippines, to Western targets. Threats are evolving with the rise of the Islamic State, which has conducted extensive recruitment in Indonesia’s national language and in the Malay language widely spoken in the region. Though the number of Southeast Asians who have traveled to the Middle East to fight with the Islamic State is considerably lower than numbers from other regions, observers estimate that hundreds of Southeast Asians have joined the fight, raising concerns that battle-trained individuals may return to the region and conduct attacks. Southeast Asia’s borders are comparatively porous, raising concerns about trans-border threats that may lead to attacks in third-party states, such as Singapore. This raises the issue of border controls, an important factor for addressing terrorism. Governments in the region have sought better coordination and intelligence sharing. These efforts have been supported by the United States. The Trump Administration has indicated that combatting terrorism broadly, and IS specifically, is among its highest foreign-policy priorities. This has implications for numerous other U.S. interests, as U.S. policy towards the Asia-Pacific region balances a wide range of security and economic goals. The United States has offered counterterrorism assistance to several Southeast Asian nations. These include helping Indonesia create a centralized antiterrorism unit and providing U.S. troops on the Southern Philippine island of Basilan to help the Armed Forces of the Philippines combat violent groups in the country’s deep South.