Medieval Jewish law forbade the practice of concubinage. Yet documentary sources from Fatimid and Ayyubid Cairo (969-1250 CE) suggest that concubinage was common in the Egyptian Jewish community. The Cairo Geniza, the worn manuscript depository of the Ben Ezra synagogue in Old Cairo, has preserved documents including family letters, rabbinic responsa, and legal sources that attest to the practice of concubinage. These records raise a range of questions about the purpose and role of illicit Jewish concubinage in its Islamicate setting and about the experiences of the concubines themselves. Why did Jewish men take concubines in defiance of communal norms and legal rulings and, sometimes, to the clear detriment of their wives and children? Historians have long presumed that instances of Jewish concubinage can be explained by Islamic influence, positing that the Muslim majority’s legal and social practices shaped Jewish practice. This paper advances another explanation for illicit concubinage among Jews: such concubinage must be understood as part and parcel of the politics of the household. Concubinage, and ownership of female domestic slaves in general, provided men with a means to assert their mastery vis-á-vis their wives and families, a means conveniently located in a legal gray area. Because concubinage lacked legal recognition, communal authorities had fewer tools at their disposal to regulate its practice. Furthermore, some medieval rabbis treated master-slave concubinage more leniently than others in their rulings. The failure of communal authorities to regulate concubinage consistently and effectively had unintended consequences. For free women married to men who took concubines, illicit concubinage could lead to increased marginalization. For the concubines themselves, the extra-legal status of concubinage arrangements both created opportunities and increased their vulnerability to their masters’ caprice. This paper analyzes documentary sources and rabbinic responsa from the twelfth and thirteenth centuries in order to recover and describe the experiences of the masters, slave women, and free wives who were impacted by illicit concubinage arrangements.