The alliance of the Roman Empire with the emerging orthodox Christian church in the early fourth century had profound consequences for the large population of Greek- (and Latin-)speaking Jews living across the Mediterranean diaspora. No known writings survive from diaspora Jews. Their experiences must be gleaned from unreliable accounts of Christian bishops and historiographers, surviving laws, and limited material evidence—synagogue sites, inscriptions, a few papyrus documents. Long neglected by historians, the diaspora population, together with its distinctive cultural forms, appears in decline by the early seventh century. This book explores why. In part, diaspora Jews suffered from disasters that affected the whole late antique Mediterranean population—continuing warfare, earthquakes, and plague. But, like all other non-orthodox Christians, Jews were subject to extensive pressures to become orthodox Christian, which increased over time. Late Roman laws, sometimes drafted by Christian lobbyists, imposed legal disabilities on Jews that were relieved if they became Christians. Fueled by malicious sermons of Christian bishops, Christian mobs attacked synagogues and sometimes Jews themselves. Significantly, Jews retained many of their earlier legal rights while other non-orthodox Christians lost theirs. In response, some Jews became Christians, voluntarily or under duress. Some probably emigrated to escape orthodox Christian pressures. Some leveraged political and social networks to their advantage. Some violently resisted their Christian antagonists. Jews may occasionally have entertained the possibility of divine messianic intervention or embraced forms of Jewish practice that constructed tighter social boundaries around them—an increased use of Hebrew, and heightened interest, perhaps, in rabbinic practices.