Constitutions are fundamental sources of authority in the states that adopt them. Yet, many constitutions offer little guidance about who is eligible for citizenship and what it means to have citizenship. This vagueness often gives rise to fierce contestation about the boundaries of membership in some constitutional states. In the essay that follows, we do not attempt to resolve this contestation. Instead, we distinguish citizenship from other forms of membership, offer an overview of the concept of citizenship, and specify citizenship’s relationship to constitutional theory. We discuss the theories that undergird citizenship practices, the norms that guide its administration, and the ways in which boundaries are established in order to delimit citizenship. We also highlight common boundary problems generated by democratic citizenship even when it is explicitly defined by established constitutional jurisprudence and describe how these boundary problems create complicated challenges that citizens, non-citizens, and states must navigate.