Arguably one of the most successful planning concepts of the Dutch Randstad is the mainport Policy which began in the 1980s. This policy proposed national advocacy for economic growth through the support for the upgrading of the infrastructure of the port of Rotterdam and Amsterdam Schiphol Airport. Dutch (public) support for the hub and transit functions of the mainports continued to weaken, more so since 2010. This chapter traces the political history of the notion of the mainport, from its conception into discourse and how it relates to our understanding of the Randstad. Since the 1980s the national spatial-economic planning policy with regard to Randstad Holland has been very much interlinked with a so-called ‘mainport’ or ‘gateway strategy’. The various initiatives have already culminated in a mutual Maritime Capital of Europe Agenda endorsed by the Rotterdam Maritime Board that represents executives from the wider port community in order to create momentum for Rotterdam as a dynamic port-city ecosystem.