The Moravians were a mostly German Pietist religious group that spread throughout the Atlantic world and beyond in the 18th century. Though considered “Protestant,” their origins predate the Reformation. In the late 14th century, a grassroots religious renewal movement began in Bohemia and Moravia that gained momentum after the martyrdom of its two most important leaders, Jan Hus (b. c. 1369–d. 1415) and Jerome of Prague (b. 1379–d. 1416). Thereafter, a mass movement developed that armed itself and successfully fought off numerous crusades by forces of the Holy Roman Empire bent on its destruction. After a settlement that secured its existence, a branch of this “Hussite” movement became pacifist and called itself the Unitas Fratrum, a name the Moravians carry to this day. Victorious imperial Catholic forces destroyed them and other “Protestants” in Bohemia and Moravia during the Thirty Years’ War, forcing them to go underground. In 1722 a remnant of the old Unitas Fratrum from Moravia settled on the estates of Count Nicolaus Ludwig von Zinzendorf (1700–1760) in Upper Lusatia (Saxony). They began building a new community called Herrnhut, with Zinzendorf as their leader, and in 1728 the Unitas Fratrum formally celebrated its rebirth. Under Zinzendorf’s direction, the movement expanded rapidly in the mid-18th century and developed a rigorous mission program that continues to this day. The Moravians promoted ecumenism in a confessional age, which led to their involvement with Lutheran, Calvinist, and other churches in often controversial ways. They are important to Atlantic history because they engaged with Europeans, Africans, and Native Americans in significant ways throughout the Atlantic world, and they kept detailed records of their activities. Many of their early missionary efforts failed, but they became noted for their successes, especially among slaves on St. Thomas, St. Croix, and elsewhere in the Caribbean; the Mahicans, Delawares, and Shawnees in British North America; Maroons and later slaves in Suriname; and Inuits in Greenland. They also had significant short-term successes among the Arawaks in Berbice and Cherokees in northern Georgia. Suriname became a long-term success story in the 19th century, and in the late 19th and 20th centuries, Moravians had tremendous success in Africa. Today, the largest numbers of Moravians are in Africa and North America, not Europe. It is their mission successes in so many places, combined with their disassociation from European imperial projects, their record keeping, and their cosmopolitan Weltanschauung, that make them such an important people to the study of Atlantic history, especially for historians who wish to cross imperial boundaries and study encounters among all peoples in the region.