What are the origins of Ottoman Islam in the 15th century? From what soil did it grow, and what nourished its development? This study follows the lives and ideas of the Yaz?c?o?lu brothers Mehmed Yaz?c?o?lu and Ahmed Bican, Sufis of the frontier city of Gelibolu and authors of the most popular religious writings in Ottoman Turkish.It places the Yaz?c?o?lus’ durable religious vision within their dynamic historical moment on the contested Ottoman borderlands. Examining how these non-elite writers deployed their own intellectual resources, it considers how they approached the religious sciences of the wider Islamic world, and how they created a religious synthesis appropriate for their own community, the growing Turcophone Muslim population of the Balkans and Anatolia.