The right to self-constitute and to demarcate the demos, i.e. the right to regulate membership, is an important part of Indigenous Peoples’ self-determination. The right to self-constitute may create different kinds of conflicts with the states in which Indigenous Peoples live but may also affect the balance of power within Indigenous communities. The transnational Indigenous Sámi people have no common constitution defining who belong to the people. In Finland, Norway and Sweden, Sámi membership has been handled in relation to specific legislation on Sámi issues. In this chapter, the authors’ aim is twofold: First, to describe the criteria for registering in the Sámi electoral roll in the Nordic states and second, to analyze why these criteria have been perceived in such a different fashion within the Sámi communities in their respective countries and why they have been implemented differently by the three Sámediggis. Since Indigenous self-determination within the borders of already-existing nation-states always means some external control and not total autonomy, it is important to compare how the question of membership is playing itself out among the same Indigenous People but in different national contexts that otherwise have so much in common.