This dissertation examines Arabic and Syriac writing on conversion among Muslims and Christians in early Islamic Greater Syria (Bilād al-Shām) and northern Iraq (al-Jazīra). It analyzes how conversion is written about and understood by scholars in these regions from 640-850 C.E., and the evolution of this understanding over the first two hundred years of Muslim rule. It does so using seventh-, eighth-, and early ninth-century Arabic and Syriac historical, legal and polemical texts that address conversion. I conclude that writing on conversion among Christian and Muslim scholars increased in sophistication and polemical focus as conversion to and from Islam increased, and religious leaders in turn constructed social and theological boundaries, bolstered by the development of law and ritual around conversion, delineating their respective religious communities. The term “conversion” is also examined in light of these changes. It is found to be largely inapplicable to Muslim and Christian writers’ understanding of religious change in the seventh-century Islamic context, and somewhat more applicable to such events in the eighth and early ninth centuries, though still not perfectly so. This research is the first comprehensive examination of conversion in early Islamic Syria and the Jazira, and expands Islamic historians’ limited understanding of conversion in early Islam. It lends insight into the poorly-understood phenomenon of Islamization of the eastern Mediterranean region during the first few centuries after the Muslim conquests, and will also contribute to historical studies of conversion in the pre-modern era, which are skewed heavily toward the study of conversion to Christianity.