Ernst-Joachim Mestmacker reviews Richard Posner's and Friedrich A.von Hayek's legal theories. Both are famous for their contributions to law and economics. They are, however, adversaries in their concepts of law and how it is to be informed by economics. Posner finds the only scientific legal theory in the external (economic) analysis of law. With Friedrich von Hayek the role of rules of conduct and legislation is to be determined by the principles that govern a free and competitive order. There are, contrary to Posner, important contributions from legal scholarship, legal history and comparative law.