Birthright citizenship refers to the legal status of citizenship when acquired through birth to a citizen parent (ius sanguinis) or birth in the territory of a state (ius soli). This is how most people acquire citizenship, often unconditionally and automatically at birth. A minority across the globe acquire citizenship through naturalization. Historically ius soli predominated from the Early Modern period, when those born in the sovereign’s territory automatically became their subjects. Ius sanguinis arose following the French Revolution, reflecting the free citizen father’s right to pass citizenship on to his child. Both forms spread globally through imitation and colonization. All states now award citizenship by birth; most have a combination of the two forms. But the strength of provisions varies. All states have substantial ius sanguinis provision; fewer have strong ius soli. In both, acquisition may depend on certain restrictive conditions related to parental birthplace or residence, marital status, gender, religion, ethnicity, or race. Until recently citizenship has been studied more by lawyers than political scientists, and birthright citizenship has received less attention than naturalization. Studies have tended to focus on the citizenship laws and policies of a limited number of states, mainly in the Global North. Only recently have studies covering a greater number and diversity of countries begun to emerge. Comparative scholars have sought to identify and explain different patterns of birthright citizenship provision related to the strength of ius soli and ius sanguinis. These have been interpreted variously as alternative models reflecting different national conceptions of citizenship, as determined by civil or common law traditions, or as dependent on histories of emigration, immigration, and colonization. Contemporary changes have been understood as a function of domestic electoral politics, developments in international law, norm diffusion among states, or a range of contingent contextual factors. Scholars dispute whether diversity of citizenship regimes has been succeeded by convergence. More complex typologies and indices, including birthright citizenship, have emerged, along with increasing availability of data on citizenship around the world. The justification of birthright citizenship has been much debated. Birthright citizenship has been seen as an appropriate way of allocating democratic membership, providing intergenerational continuity of citizenry, reducing the incidence of statelessness, and integrating immigrants. But ius sanguinis has often been criticized as exclusive and illiberal. It is debated whether ius soli is better justified, or if all forms of birthright citizenship should be seen as conveying arbitrary privilege and contributing to global inequality.