Also labeled undocumented, irregular, and unauthorized migration, illegal migration places immigrants in tenuous legal circumstances with limited rights and protections. We argue that illegal migration emerged as a structural feature of the second era of capitalist globalization, which emerged in the late twentieth century and was characterized by international market integration. Unlike the first era of capitalist globalization (1800 to 1929), the second era sees countries limiting and controlling international migration and creating a global economy in which all markets are globalized except for labor and human capital, giving rise to the relatively new phenomenon of illegal migration. Yet despite rampant inequalities in wealth and income between nations, only 3.1 percent of all people lived outside their country of birth in 2010. We expect this to change: threat evasion is replacing opportunity seeking as a motivation for international migration because of climate change and rising levels of civil violence in the world’s poorer nations. The potential for illegal migration is thus greater now than in the past, and more nations will be forced to grapple with growing populations in liminal legal statuses.