Drawing on leaked documents and other original sources in Arabic, this article examines the internal struggles over defining the Islamic State’s ideology during the period 2014 to the present. Since the Islamic State declared the caliphate in summer 2014, disagreements over doctrinal matters—primarily related to takfir (excommunication)—have sparked furious debates among different factions of the group. The earliest bout of infighting culminated in 2014 in the execution of a number of “extremist” scholars and activists. Infighting would reemerge in 2016, however, and grow increasingly more contentious, leading to the release of dueling pronouncements on takfir, the dismissal of numerous officials in the Islamic State’s executive council, and later their defection and flight from the group. The disaffected include both those who believe the group has become too extreme and those who believe it has become too moderate. The ideological incoherence in the Islamic State may well affect its long-term prospects.