Examining sectarian divergence in the early modern Middle East, this study provides a fresh perspective on the Sunni-Shi'i division. Drawing on Ottoman Turkish, Persian, and European sources, Ayşe Baltacıoğlu-Brammer explores the paradox of an Ottoman state that combined rigid ideological discourses with pragmatic governance. Through an analysis of key figures, events, periods, and policies, it reveals how political, economic, and religious forces intersected, challenging simplistic sectarian binaries. Baltacıoğlu-Brammer provides a comprehensive historical account of Ottoman governance during the 'long sixteenth century', focusing on its relationship with non-Sunni Muslim subjects, particularly the Qizilbash. As both the founders of the Safavid Empire and the largest Shiʿi-affiliated group within the Ottoman realm, the Qizilbash occupied a crucial yet often misunderstood position. Boundaries of Belonging examines their role within the empire, challenging the notion that they were merely persecuted outsiders by highlighting their agency in shaping imperial policies, negotiating their status, and influencing the Ottoman-Safavid rivalry in Anatolia, Kurdistan, and Mesopotamia.