In this thesis I try to defend the concept of residence-based citizenship. My point of departure is the puzzling observation that, in the 19th and the beginning of the 20th centuries, a quite large number of countries practiced ius domicilii and unconditional ius soli as the most important principles of citizenship acquisition, against a growing number of states following the 1804 French Civil Code which reinvented ius sanguinis. In less than one hundred years however, ius sanguinis became the most important principle of citizenship acquisition all over the world, ius soli was largely restricted, and ius domicilii almost disappeared. My intention is not to investigate this historical process, but to explore the ways in which normative theories and academic research in immigration studies may reveal the need to re-evaluate a residence-based citizenship theory based on ius domicilii. In this sense I am analysing four test cases which have in common the essential fact underlined by Joseph Carens that in time immigrants become members of society, irrespective of political authorities’ decisions. This is enough to substantiate a claim to citizenship. The test cases are irregular migrants, temporary workers, dual citizens and 'external quasi-citizens.’ I argue that the 'undocumented’ have a moral claim to regularisation after one year of illegal residence, and further that the first two categories of migrants have a moral claim to citizenship acquisition after three years of legal residence, which is the threshold supported today by a few liberal states. But if residence supports a claim to citizenship, then lack of it for an extended period sanctions loss of this status. Thus I argue against dual citizenship, trying to explain that its advantages are either rather imagined than real, or they can be achieved through more convenient means. However, in order to acknowledge the fact that many people have real and strong ties to more than one country I suggest the status of 'external quasi-citizenship’ which may be accessed by non-residents and may provide numerous entitlements, possible all citizenship rights except the right to vote and the right to stand for elections. I also analyse legal provisions in fifty states and try to make a normative plea for residence-based citizenship. In a final step, I discuss four alternative theories of citizenship which do not easily come to terms with the idea of residence-based membership and try to make a case for the latter.