This chapter introduces the book and gives a brief overview of each chapter. Citizenship is no longer an exclusive relationship. Nowadays, many people are citizens of two, or even more, states. Sometimes that occurs through an accident of birth or a decision to move to a new country and to take out citizenship there. Other times a second citizenship is almost literally purchased, through schemes fast-tracking citizenship applications from foreigners making large investments in the state. There are moral problems with each way of acquiring a second citizenship, as there are with retaining one's original citizenship when acquiring another. Over and above issues with how it comes about, multiple citizenship also has morally troublesome consequences for the coherence of collective decisions, for the constitution of the demos, and for global inequality. Many of these problematic features of multiple citizenship could be avoided by unbundling the rights that currently come with citizenship and allocating them separately from one another.