The present work deals with Islamic law and the law of the Muslim countries. More specifically, the author has studied the constitutional law of the Islamic legal tradition, starting from the perspective of the law of Caliphate. Moreover, this work focuses on the process of institutionalization, which, since the first half of 19th century, led to the adoption of civil law in both sunnī and šī‘ī world. Further, this study combines different methodological approaches, based on Islamic law, analyzing sources in Arabic, especially classical and contemporary fiqh booklets, and also following the approach of comparative public law to highlight how modern Constitutions of Muslim countries differ from western constitutionalism.