Understood simply, people are either citizens of a country or stateless. Yet reality belies this dichotomy. Between absolute statelessness and full citizenship exist millions of people who are nationals of a country in principle but lack the identity documents to prove it, beginning with a birth certificate. Languishing in a gray zone, undocumented nationals have difficulty accessing the full services and rights that their documented counterparts enjoy. Drawing on a range of country examples, Undocumented Nationals: Between Statelessness and Citizenship calls attention to and analyzes the plight of people who cannot exercise full citizenship owing to evidentiary deficiencies. The existing literature has not adequately conceptualized and examined this in-between status, which results sometimes from state neglect and other times from intentional state discrimination. By highlighting its causes and consequences, and exploring ways to address the problem, this Cambridge Element addresses an important gap in the literature.