Ships are no Flying Dutchmen! They need a harbour. Therefore, the development of ship construction is pretty much connected with that of harbour construction, and beyond this, they influence the topography and infrastructure of a harbour. The transition between the Medieval period and the Early Modern Age is a period of great change in the development of larger ships, even in the Baltic. Furthermore, the internationalisation of Baltic trade took place. In Medieval times, ship construction followed conditions in the harbours. In the Early Modern Age, it was the other way round. Now, harbour construction, topography and infrastructure follow the development of ship construction. The paper focuses on the deep impact that larger multi-mast sailing ships had on the development of Baltic harbours.