This is the first volume to attempt a comprehensive overview of the evolution of the 'Arian' churches in the Roman world of Late Antiquity and their political importance in the late Roman kingdoms of the 5th-6th centuries, ruled by barbarian warrior elites. Bringing together researchers from the disciplines of theology, history and archaeology, and providing an extensive bibliography, it constitutes a breakthrough in a field largely neglected in historical studies. A polemical term coined by the Orthodox Church (the side that prevailed in the Trinitarian disputes of the 4th century C.E.) for its opponents in theology as well as in ecclesiastical politics, Arianism has often been seen as too complicated to understand outside the group of theological specialists dealing with it and has therefore sometimes been ignored in historical studies. The studies here offer an introduction to the subject, grounded in the historical context, then examine the adoption of Arian Christianity among the Gothic contingents of the Roman army, and its subsequent diffusion in the barbarian kingdoms of the late Roman world.