Abstract: This paper compares the conception of the rule of law in U.S. constitutional law with the corresponding conception of the Rechtsstaat in modern German constitutional law. The type of Rechtsstaat established by the German constitution is clearly substantive. A commitment to human rights is understood as inherent in the concept of the Rechtsstaat, and so is the requirement of proportionality as a standard for evaluating restrictions on personal and economic liberties. In U.S. constitutional law, by contrast, the concept of the rule of law does not operate as an enforceable constitutional doctrine, but rather as an ideal lying behind and informing both constitutional and nonconstitutional doctrines. Characterizing the vision of the rule of law expressed in U.S. constitutional law is, therefore, more difficult, but I argue that it is primarily procedural (or formal), with some substantive elements. The concept of proportionality does not lack parallels in U.S. constitutional law; basically, it is a form of balancing of interests. But balancing is not regarded in U.S. constitutional doctrine as an element of the rule of law, and it is not applied to interferences with all constitutional rights. This is especially true with regard to economic rights that are more highly protected in German constitutional law. The German constitution has been very influential as a model of constitutionalism, and one of its most exportable features may be its image of the Rechtsstaat. It remains to be seen to what degree the more substantive conception of the Rechtsstaat will carry protection of economic rights under the proportionality principle more widely into national constitutions and transnational arrangements, and to what degree those institutions will content themselves with the thinner Anglo-American conception of the rule of law.