Every year, millions of Muslim men from around the world gather by the banks of the Turag River in Tongi, Bangladesh, for a three-day celebration of Islam. The annual event, known as Bishwa Ijtema (Bengali for "world gathering), is organized and hosted by a group known as Tablighi Jama'at (Proselytizing Group). Although the group is little-known outside the Muslim community, it has one of the largest followings in the world. In 2008, it was estimated that some two to three million people attended this celebration of Islam, and in 2010, local police estimated the number had reached five million. Tablighi Jama'at is a Muslim missionary and revival movement that is as enigmatic as it is unique. Combining a strict, traditional approach to orthodox religious practice with loose, decentralized governance over those who follow it, the group is shrouded in secrecy. This makes Tablighi Jama'at highly controversial, and the study of its ideology and practices, as well as the web of its potential role in the world of jihad, difficult to unravel. In the years since its founding in the mid-1920s, Tablighi Jama'at has become the largest group of religious missionaries in the world, with a presence in some 80 countries from Bangladesh to the United States. Because there is no formal census of its members, who join and leave the group at will, its numbers are loosely estimated to be anywhere from 12 to 80 million followers. Its central mission is to send its emissaries around the world to proselytize to non-observant Muslims and help them return to the more orthodox traditions of Islam. They avoid contact with non-Muslims whenever possible. Yet despite its size, its global reach, its lack of strict, top-down controls on its millions of followers and the scope of its activities is a closely held (and for the most part, well-kept) secret. The organization reveals only the bare bones of its philosophy and its mission to the public. Its message is contradictory, at least from a Western perspective, and this has formed the basis for considerable speculation, not to mention a widely held suspicion about its possible connections to terrorism. The group professes to follow strict Muslim doctrine, but to shun violence. Tablighis claim to have no formal organization, no paid staff, no official membership, no property except that which is gifted to them, and sustain their global networks of followers through person-to-person contact. Whether they have ties to terrorism, directly or indirectly, is a topic of ongoing discussion. The indicators are ambiguous at best. Is this a coincidence or is it by design? Does it signify a deeper, darker mission, or are any connections to terrorist activity simply coincidental? The secrecy underlying Tablighi Jama'at's activities make these questions difficult to answer. This monograph will try to clarify the ideology and activities surrounding this mysterious and powerful group.