The Islamic conquest of the Sasanian Empire inaugurated, among many other transformations, the progressive Islamization of the region. The pace and mechanisms of this transformation remain poorly understood. No demographic data survives from early Islamic Iraq, and the question of Islamization did not much exercise authors from that period. Yet the progress of Islamization in the capital province of the Abbasid caliphate is a significant hidden variable in the study of Muslim relations with non-Muslims and the Abbasid state’s interactions with its subject populations. One scholar estimated that Abbasid rule witnessed the change from a small Muslim minority in Iraq (16% in 791) to a large supermajority (84% converted by 976), and a large number of studies have proposed various motivations and mechanisms for mass Islamization during the early Abbasid period. This paper will adopt a spatial approach to Islamization, looking for differential developments in different areas within Iraq, with particular sensitivity to the distinctions between newly founded and pre-Islamic cities, and between urban and rural society. The study is based on a comparison of Muslim geographical sources (such as al-Balādhurī, Ibn Ḥawqal, and al-Muqaddasī) with Christian sources in Syriac and Arabic (including Ishoʿdnaḥ of Baṣra, Eliyā b. ʿUbayd al-Dimashqī, and ʿAmr b. Mattā), as well as archaeological evidence where available. Rather than attempting a quantitative approach on such uneven data, this paper will offer a rhetorically sensitive reading of relevant literary passages, anecdotes which often unself-consciously reveal what each author presents as normal or unusual. These readings will synthesize not only reports of multi-religious presence, but also the changing distribution of the ecclesiastical hierarchy and monastic networks, as well as the spread of mosques and changing uses of religious buildings. This paper will argue that Islamization was more rapid in southern Iraq than in the north, but as late as 950, substantial areas of the countryside had been only lightly influenced by Islamization. This suggests that we must explain mass Islamization by forces relevant to the period of Abbasid disintegration or later, rather than to that of Abbasid dominance, and certain aspects of Morony’s thesis on the continuities between Sasanian and early Islamic Iraq may be extended to the end of the first millennium. Rather than replacing an Arab nationalist story with an Iranian, this paper suggests the importance of considering Jews and Christians as links between the multi-religious and multi-ethnic Sasanian Empire and the Abbasid caliphate.