This paper will discuss boundaries between Muslims and non-Muslims in Ottoman society (17th to early 19th centuries). It will use findings of recent studies on responses to natural disasters to demonstrate that on many levels, social borders between Muslims and non-Muslims were rather loose. Recent studies have shown that the previously prevailing assumption that reactions to natural disasters, and particularly plague, were tied to one’s faith, is no longer valid. That is, Muslims did not entirely refrain from fleeing plague-ridden areas, despite injunctions not to do so found in Islamic plague treatises; and flight was not always an obvious choice for Jews and Christians. The Ottoman state, too, did not usually discriminate against non-Muslims in its allocation of resources when offering post-disaster relief. Because natural calamities happened so often in the Ottoman Empire and were virtually an inseparable part of people’s lifetime experiences, such findings have broader implications for the study of Ottoman society. Combined with new evidence from Ottoman, French, and British archives, Arab chronicles, and Jewish responsa literature, they suggest that Jews and Christians living in the Ottoman Empire were not organized as segregated communities, but rather well integrated into Ottoman society and the Mediterranean commercial scene. Jewish communities, in particular, should be described as fragmented social units that interacted freely in a public sphere that integrated many other groups, and that lacked formal, top-down leadership. Therefore in this paper, which is based on a major ongoing research project, I will argue that Jews and Christians living in Ottoman cities had more in common with Muslims than historians have assumed; and that confessional boundaries, previously believed to have been imposed by the Ottomans and communal leaders, were in fact rather porous and played little part in the daily activities of most Ottomans, Muslim or non-Muslim. This only changed in the mid-nineteenth century with the advent of religious reforms as part of the Tanzimat.