This book explores the widespread mass conversions to Christianity and Islam that took place in Europe and Asia in the ninth to eleventh centuries. Taking a comparative perspective, contributors explore the processes at work in these conversions. Focusing on Christianity and Islam, it contrasts religious conversion in the period with earlier conversions, including those of Manichaeism in central Asia; Buddhism in east Asia; and Judaism in Khazaria, exploring why conversions to Christianity and Islam led to centralized political structures.