This chapter deals with a topic not very often discussed in the study of customary international law (CIL): the use of customary international norms in the internal legal order of a state. The use of these norms (regulating the relationship between states) is particularly relevant for federal states. The chapter studies the use of CIL norms in two federal states: Germany and Austria. In Germany, CIL norms became part of the domestic legal order through the 1919 Constitution. German courts used those norms to settle disputes between German states in those cases where the federal framework did not offer clear solutions. Under the 1949 Basic Law, the Federal Constitutional Court has been inconsistent on the question whether the constitution allows for the use of customary international norms to settle disputes among German states, but at the very least it uses norms that are materially identical to customary norms in international law. In Austria, the federal constitution expressis verbis acknowledges the use of international norms in the domestic legal order to regulate interstate relations. It is not entirely clear however whether these norms are of a customary nature, or are simply those found in the Vienna Convention.