Dual or multiple nationality/citizenship is a status that grants an individual membership in two or more states. This status was repudiated and fought against legally and culturally, but it has been normalized since the end of the 20th century as a result of various changes that occurred in the aftermath of World War II: (1) decrease in international conflict and a reduction of compulsory military service; (2) development of human rights and gender equality, allowing women to transfer their nationality to their offspring; (3) increase in international support for the prevention of statelessness; and (4) increase in international migration and intermarriage. An individual can become a dual/multiple national/citizen by birth or by naturalization. In international law, nationality and citizenship are used interchangeably, however, some countries draw legal distinctions between them; moreover, various social scientists insist on distinguishing between these two closely related concepts. While countries may legally accept or reject dual nationality/citizenship, the reality is more complex because there is formal and informal tolerance of this status. This tolerance can also be differential (e.g., restriction of dual nationality/citizenship via naturalization and tolerance of this status when individuals are born in the territory and inherit a second—or more—nationality/citizenship from their parents). Dual or multiple nationality/citizenship can also diverge in its origins and consequences depending on whether it involves immigration or emigration states and in the degree to which dual nationality/citizenship is granted (e.g., acceptance of the retention of nationality when emigrants nationalize abroad while restricting their access to citizenship rights, such as political rights). The increase in dual nationality/citizenship since the late 20th century has promoted a normative debate (more intense, initially) about its consequences in terms of military service, state loyalty, diplomatic protection, equality of rights among citizens, and so on. However, thanks to the proliferation of comparative and single studies of dual nationality/citizenship around the globe, we may now analyze not only the reasons that brought about the acceptance, rejection, or tolerance of this status but also its practical consequences. Scholars have studied the effects of dual nationality/citizenship in many areas, such as international relations, nationalism and the state, migrants’ integration in countries of reception, membership and rights extension to migrants in countries of emigration, political participation, instrumental use of this status, and so on. While the causes and consequences of dual nationality/citizenship vary widely, some regional patterns around the globe have been identified.