This paper examines the explanatory and the prescriptive value of German (and related) federalist ideas with regard to the constitutionalization of international law. The author contends that respective scholars have, on the one hand, developed federalist thought with regard to the national constitutional level which may help to explain or shape international processes of constitution-building. On the other hand, they have themselves promoted international federalism as a natural extension of their national constitutional doctrine, hence partially weakening the classical dichotomy between national and international law.