This introductory chapter provides an overview of the book's main themes. This book addresses three key issues that are central to the current politicisation of migration and citizenship, but that take particular forms in the United Kingdom and the United States. These three issues, each of which is discussed in a separate chapter, are borders and walls, mobility, and belonging. Borders and walls are the material articulation of state boundaries and state sovereignty. The role of borders is increasingly seen as the regulation of the movement of people: facilitating easy movement across borders for some, but making that movement more difficult for most. In contrast, the mobility of capital, through the actions of transnational corporations in particular, receives little attention.