Many political theorists assume that the openness of immigration and the inclusiveness of citizenship trade off. Yet, there is no consistent empirical evidence for this negative relationship. This dissertation advances three papers to investigate the association between immigration regimes and citizenship regimes. The first paper introduces a new citizenship policy dataset and charts policy trends. I find a liberalizing trajectory that has stagnated, as well as long-term convergence in citizenship regimes across 23 democracies 1980-2019. In addition, I advance index methodology by introducing the idea of confirmatory dimensionality testing within a three-level approach to concept formation. The second paper maps immigration and citizenship regimes in a novel and empirically validated two-dimensional typological space across those cases until 2010. Overall, boundary regimes have become more open-inclusive and less closed-exclusive over time. The liberalizing and converging tendencies are especially pronounced in immigration regimes due to liberal constraints. Based on these descriptive analyses, the third paper develops and tests the boundary politics framework across 23 democracies 1980-2010. It shows that, as theorized, in cases in which immigration-related issues are not politicized, immigration and citizenship regimes do not correlate. When immigration is politicized, immigration regime openness and citizenship regime inclusiveness correlate positively as they become part of the same cultural dimension of party politics, yet they only do so after the Cold War. The evidence shows further that the strong liberal constraints that immigration regimes are exposed to cannot be fully suppressed even when nativists are strong, while citizenship regimes respond to nativist party power and become more exclusive even when immigration is not politicized. These empirical findings corroborate but also qualify the boundary politics framework. They also provoke some surprising implications for various ideal typical positions in normative theory. The allegedly unrealistic liberal-cosmopolitan vision of open inclusive boundary regimes emerges as the least troubled stance.