This edited volume examines the theoretical versatility of the concept of “borders.” The impulse to categorize, while present from antiquity in Western culture, has increased in intensity since the advent of the modern age with its corresponding political rise in the ideology of the sovereign nation-state. While the concept of immigration is the common mental image Westerners have when discussing borders, immigration is only the tip of the iceberg for this book. The belief in mutually exclusive, clear, and concrete categories, a necessary ideology in the age of the nation-state, creates large swathes of exceptions where people live ambiguous lives nationally, racially, sexually, ethnically, and in terms of gender. National identity, race, sexuality, gender, and the intersections between are the main categories discussed in the book through the lens of borders and ambiguity. The fervor over categorization, best embodied in recent political history by the Trump administration in the U.S., is both a desire to identify and thus control various “dangerous” populations, as well as creating the very ambiguity categorization is intended to alleviate. The volume weaves together discussions on the subjective meaning-making in ambiguity, policies that create ambiguity, historical creations of ambiguity that persist to the present, and theoretical considerations on the relationship between borders and ambiguity.