French postwar planning has been interpreted as an avatar of a French mercantilist tradition or as smart adhoc tinkering. Archival research shows the shortcomings of both interpretations. It is proposed here that Monnets plan had intellectual and structural roots in American New Deal Keynesianism. A genealogy of French planning shows economies to be politically constructed sets of institutions. The role of cross-national transfer mechanisms in such institutional building is also underscored.