This is a comprehensive historical study of the Islamic mystical brotherhoods of the northern Sudan. Based on new or previously inaccessible oral and written sources, it traces the change from lineage-based holy clans to centralized supra-tribal brotherhoods in the 19th century. It links this evolution to both external influences from Egypt and Arabia and changes in northern Sudanese society brought about by Egyptian colonial rule. The analysis of this fundamental shift in the nature of religious organization is seen as a major contributory factor in the Mahdist Revolutiuon of 1882-5. The last two chapters present an account of the structure and rituals of the brotherhoods based on their own writings.