Islam has maintained its presence in Southeast Asia for more than a millennium, dating back to as early as the 7th century. By the 21st century, the estimated total of Muslims surpassed 240 million, making Southeast Asia a site that is populated by one of the largest Muslim communities on the planet. Muslims are majorities in 21st-century Brunei, Indonesia, and Malaysia. In other Southeast Asian countries, namely Myanmar, Cambodia, Philippines, Singapore, Thailand, Timor Leste, and Vietnam, they remain as minorities, experiencing varying levels of integration and assimilation with the majority non-Muslim Buddhist or Christian populations. The massive though gradual spread of Islam in the region can be attributed to the generally peaceful, multifaceted, and creative ways by which Islam was infused into the everyday life of local societies. Traders, Sufi missionaries, scholars, rulers, and even non-Muslims have all contributed to the spread of Islam in Southeast Asia. Most Muslims in Southeast Asia are Sunnis, adhering to the Shafi’i school of Islamic jurisprudence, Ash’ari theology, and Sufi ethics. Located within this cosmopolitan and diverse religious landscape are Muslims who belong to other schools of Islamic jurisprudence and theological leanings such as Hanafi, Ja’fari, Shi’ah, and Salafi. Viewed from the perspective of the longue durée (long duration), the venture of Islam in Southeast Asia can be divided into four successive phases: gradualist (7th–14th centuries), populist (15th–19th centuries), colonial-reformist (19th–mid-20th centuries), and assertive (mid-20th–21st centuries).