Modern federations are faced with the challenge of cross-state as well as cross-nation economic activities and with the ever-increasing mobility of society. This has not only promoted international law, but has also created the need for harmonized laws throughout federations within the competence areas of the states. Diverse laws within federal systems may increase transaction costs, cause delays, and lead to jurisdictional conflicts for nationwide or cross-state transactions. In order to preserve federalism, and therefore prevent an ever-advancing process of centralization, interfederal legal harmonization promoted by the states themselves is crucial. There are two distinct methods of legal harmonization of state laws: (1) harmonization by “Uniform Law Conferences,” which are in principle run by lawyers and thus independent, to a certain extent, from the influence of policy makers; and (2) harmonization by executive intergovernmental conferences. These two distinct models of interfederal legal harmonization will be analyzed and evaluated with regard to efficiency, compatibility with democratic principles, transparency, and accountability in a comparative legal study of the harmonization processes. This Article will scrutinize the federal systems of the United States and Canada, on the one hand, as well as those of Germany and Austria, on the other hand. The study will reveal that the efficiency of interfederal legal harmonization increases with the level of intergovernmental integration through the participation of government officials and their staff.