The Islamic Republic of Iran prides itself on being the only country with an entirely codified Islamic legal system, and on being a pioneer in the Islamization of constitutional law. Part 1 of this chapter provides an overview of the different models of Islamic constitutionalism currently found in the Muslim world. Part 2 reviews Iran’s highly creative and ambitious project of Islamizing an entire civil law system and codifying Islamic law over a forty-year period and draws attention to the high degree of dynamism and reinterpretation of Shiite legal precepts that this project has required. Part 3 focuses on the making and amendment of Iran’s 1906/1907 and 1979 constitutions – which fused foreign, republican, and Islamic elements in unique ways – and on the role of the 1979 constitution in defining and regulating Iran’s distinctive present-day blend of institutional conflict and policy disagreement among religious conservatives, pragmatic reformers, and revolutionary leftists. Over time, a combination of innovative reinterpretation of Shiite legal principles and constitutional and institutional reform have reshaped the complex relationship between left-leaning legislative institutions constrained by Islamic principles and conservative religious scholars who operate outside the political system but are the arbiters of what it means to respect Islamic principles.