This dissertation examines the formation of popular Islam in the Ottoman Empire. It does so by exploring the intellectual genealogies, social context, and dogmatic program of the Yazıcıoğlu family of scholars of the fifteenth-century Mediterranean frontier city of Gelibolu. The Yazıcıoğlus, represented by the brothers Mehmed (d. 1451) and Ahmed Bican (d. ~1466), jointly composed some of the most widely-read catechistic, dogmatic, and natural-philosophical texts ever written in Ottoman Turkish. They viewed their successful literary careers as a joint effort to disseminate religious and philosophical knowledge to the newly Islamized community around them. “I wrote my works,” Ahmed claimed, “so that the people of this land of ours may gain the light of knowledge… and understand the bond of Islam.” This study of the Yazıcıoğlus thus proposes to address the basic question of how and out of what ingredients Ottoman popular piety developed. First, the dissertation uses documentary and literary evidence to assemble a biographical sketch of the members of the Yazıcıoğlu family, emphasizing their relationship to the evolving Ottoman state. Next, this study examines the intellectual heritage of their popular writings, showing the way they draw from the Hanafi schools of Timurid Iran and Central Asia, from international Sufi trends, and other sources. The following section dwells on the Yazıcıoğlus' frontier context, and how their works respond to it by attempting to define a community of normative Muslims in the fluid sectarian environment of the Mediterranean borderlands.The study then assesses the place of these Ottoman texts within the landscape of Islamic theological disputation, including controversies surrounding monist Sufism, Sunnism and Shi'ism, apocalypticism, and natural philosophy. An appendix argues that the Dürr-i Meknun, a famous Turkish encyclopedic work, is not by Ahmed Yazıcıoğlu as has been traditionally assumed. The dissertation concludes that Ottoman popular piety, as a spiritual vernacular, is an idiosyncratic adaptation of regional and cosmopolitan trends to local circumstances of the Mediterranean frontier. Popular Islam in the Ottoman Empire emerges from this confluence.