Since its first codification in the early twentieth century, Iranian family law has followed the Shiʿi (Jaʿfarī) school of jurisprudence. In other parts of the Shiʿi world, the question of codifying Shiʿi family law has emerged more recently. This chapter argues that codification enhances the formal rule of law. In the past, family law codification was considered to conflict with a fundamental element of Shiʿi legal thought and religious practice, namely ijtihād, independent legal reasoning by qualified scholars, which makes for a living law. Based on a comparative analysis of Iranian family law and recent Shiʿi (draft) laws put forward in Afghanistan, Bahrain, and Iraq, this chapter discusses where modern Shiʿi family law is located between the “opposite” poles of the formal rule of law (where law is general, prospective, clear, and certain) and ijtihād. The findings indicate that, today, the two are not viewed as contradicting each other. Yet, while Iranian family law only serves as a limited model for other parts of the Shiʿi world, the comparison shows that Iran subjects Shiʿi family law to the formal rule of law more comprehensively than is the case in the other three analyzed countries.