This chapter argues that in order to understand the linkages between populism and restrictions of migrants’ rights, more awareness is needed of the distinction between empirical facts and institutional facts. If the distinction is disregarded, negative consequences can follow. The chapter explains that mobility – the movement of bodies through space – is the empirical fact to which the law attaches a series of legal concepts by way of institutional facts (e.g. citizenship, residence, population, and migration). The chapter demonstrates that, once aware of the distinction, we are better equipped to see how legal regulations governing the institutional fact of population affect our social and constitutional identity. Migration law and policy ultimately impact on the composition of the institutional fact of the People (those who count as citizens) and its role in the constitutional order (what the citizens do, which rights and duties they have), that determines the state’s constitutional identity. Therefore, as this chapter argues, migration law can be employed to both strengthen and dismantle constitutional democratic institutions. Migration lawyers thus have much to offer to constitutional lawyers who are concerned about the contemporary resilience of the constitutional setting.