This chapter explores the nature of conversion in the early medieval Middle East by focusing on the first half of these convert martyrs, who began their lives as Christians, embraced Islam, and then returned to Christianity. Among these, there were several subgroups, including Christians who converted to Islam as slaves or prisoners, Christians who converted under disputed or contingent circumstances, and martyrs who were brought up in religiously mixed families. Because of the contingent nature of this process, conversions could also be undone, leading to sizable numbers of apostates over the course of the seventh, eighth, and ninth centuries. Even if the number of apostates paled in comparison with the number of those who converted and remained Muslims, their paths in and out of Islam tells a great deal about how conversion worked, especially the myriad social, spiritual, economic, and political pressures that powered religious change in the period.