The argument of this paper is that several empirical puzzles in the citizenship literature are rooted in the failure to distinguish between the mainly legal concept of nationality and the broader, political concept of citizenship. Using this distinction, the paper analysis the evolution of German and American nationality laws over the last 200 years. The historical development of both legal structures shows strong communalities. With the emergence of the modern system of nation states, the attribution of nationality to newborn children is ascribed either via the principle of descent or place of birth. With regard to the naturalization of adults, there is an increasing ethnization of law, which means that the increasing complexities of naturalization criteria are more and more structured along ethnic ideas. Although every nation building process shows some elements of ethnic self-description, it is difficult to use the legal principles of ius sanguinis and ius soli as indicators of ethnic or non-ethnic modes of community building.