One of the most enduring sources of conflict among Muslims is the question of who or what represents legitimate power and authority after the Prophet Muhammad. This introduction briefly examines the diverse answers that key representatives of the classical Islamic tradition offered to this controversial question. A concise overview of early Islamic political history is followed by a survey of Islamic thought on the subject of authority in the formative (seventh-ninth centuries) and classical periods (ninth-thirteenth centuries). This introduction presents the views of six major theological schools of the classical period: Ashʿarism (representative of Sunnism), Muʿtazilism, Ibadism, Twelver Shiʿism, Ismaʿilism, and Zaydism. Finally, this chapter discusses the classical Arabic texts that appear in English translation in this anthology as well as their respective themes, authors, and historical contexts.