This research focuses on the problem of the incorporation of international law into the German Law. International treaty is incorporated into German Law by statute passed by the legislative organ. However, the legislative organ can authorize the excutive organ to incorporate international treaty into municipal law lay making administrative order. International custom is incorporated into the German Law by Article 25 of the Constitutional Law. The hierarchical rank of the incorporated international law in the German Law sphere varies, depending on the rank of the norm which incorporates it. There are two different theories about the nature of the incorporated international law: transformation theory and adoption theory.