This book contends that the transition of leadership from Ayatollah Ali Khamenei will result in a crisis of legitimacy for the Islamic Republic of Iran. Using Max Weber's typology of legitimacy, the book explains that the Islamic Republic's legitimacy was based on the charismatic authority of the regime's founder, Ayatollah Ruhollah Khomeini. Since Khomeini's death in 1989, the regime has failed to develop the rule of law necessary for legal-rational authority. Moreover, it abandoned the logical underpinnings justifying clerical rule when a mid-ranking cleric rather than a Grand Ayatollah was placed in the position of Supreme Leader. With neither a legal basis nor a traditional basis of authority, the new leader relied extensively on the cover of Khomeini's charismatic shadow for legitimacy. After nearly four decades, this shadow is fading. Not only will Khamenei's successor lack the same direct ties to Khomeini, but the demographic and societal changes in Iran have made the charisma of Khomeini a historical concept rather than a viscerally felt experience. First the book analyzes the likely succession scenarios, finding the most probable outcome is the appointment of a hardline conservative backed by the regime's security forces. Next, the regime's economic, political, and social failures are presented, in order to explain why the new leader is likely to try to return to a traditional basis of legitimacy - religion. Thereafter, the book explains how this hardliner focus on religion is likely to result in an aggressive Iranian foreign policy toward the United States, Israel, and Saudi Arabia, impacting the region's security.