In sheer numbers, South Asia is the global center of Islam. It is the host to over 350 million Muslims: 128 million in India, 124 million in Pakistan, and 103 million in Bangladesh. Muslims have been present in South Asia for almost as long as they have been present in the Arabian peninsula. The first mosque in South Asia was built in Kerala not long after the death of the Prophet Muhammad in 642. Three factors led to the growth of Muslim communities in South Asia: commerce, conquest, and conversion. This article focuses on Islamic communities in South Asia and their history. It discusses early contacts and coastal communities of Muslims, Isma'ili syncretism and maritime trade, the resurgence of the Sunnis and empire building, Sufi orders and religious fluorescence, Shi'i communities, Islamic cosmopolitanism and conversion in Mughal times, colonial expansion and Muslim reaction, independence, communalism, partition, religious communalism and radicalism, and transnational movements in the age of globalization.