Preface If we can measure the success of a political concept by the frequency with which it is used in speeches and texts, then ‘citizenship’ has been hugely successful. A quick search on Google Scholar shows 1.87 million academic texts where the English version of the concept comes up. The use of the concept in all English language texts registered by Google peaked first around 1920, declined then continuously until the early 1980s, and has since risen steeply to a new record frequency around the turn of the millennium. Since so many authors talk about citizenship the content associated with the concept is likely to become less precise. Citizenship is indeed used today in many contexts where it would have seemed oddly out of place in the past. Big companies advertise their ‘corporate citizenship’, users of internet-­ based social networks are addressed as cyber citizens and political philosophers and activists campaign for animal citizenship. The present volume sticks to the core meaning of citizenship since the times of the Athenian and Roman republics as a status of equal membership in a political community. Since the French and American Revolutions, citizenship has become most closely associated with nationality: a legal status that attributes responsibility for individuals to states and a bundle of rights and obligations assigned to individuals by states. The broad new literatures on citizenship of minorities or on citizenship as a practice of contestation in social movements generally assume national citizenship as a stable background. They discuss inequalities and exclusions among those who share the same legal status and rights of national citizenship. This is an important agenda, but it needs to be broadened by examining how the presumptively stable background of national citizenship itself is gradually shifting. This question drives the four debates collected in the present volume. The first of these raises a question that is not entirely new in historical ­perspective, but that has gained new salience in contexts of globalisation, mobility and rising global inequalities of income and wealth: Should citizenship be for sale? And does the sale of passports to investors change the very nature of citizenship by turning it from a stable legal bond into a marketable commodity? The second debate examines another phenomenon that had been common in ancient and medieval republics: banishing citizens from the polity as punishment for law-breaking or as a precautionary removal of those who vii