Guests and Aliens shows the causes of immigration that historically have resulted in nations welcoming incomers as guests or disparaging them as aliens. Sassen describes the relative normality of the pursuit of work across borders during the emergence of the European nation-states and explains the economic and political mass migrations of Italians and Eastern European jews during the nineteenth and early twentieth centuries.

She also discusses the dislocations - particularly those after the end of World War II - that have engendered the refugee concept. By mapping the long history of global migration, Sassen shows that the American experience is just one phase in an extended history of border crossing.