Municipal park systems played an important role in the comprehensive city plans that were developed on both sides of the Atlantic at the beginning of the twentieth century, for example in Washington D.C., Chicago, Berlin and Rome. Using these cities as examples, the article shows how open space planning and design crossed national borders. Park systems in the USA attracted attention in Europe where they were regarded as models that could facilitate comprehensive city planning. Whereas the systematic, efficient and rational American approach using nature to order and direct the social and spatial development of entire cities had a wide audience amongst city planners, landscape architects and architects in Europe, European garden art provided design languages and other cues for planners and landscape architects in the USA. Besides being an international phenomenon and one of the bases for transnational urbanism at the beginning of the twentieth century, park system planning - due to its dependence on natural features and ecosystems - also inspired the planning disciplines to cross political boundaries. In all four cities, the municipal park system, intended to improve the urban living environment and to signify the respective city's modernity, became the precursor of regional planning schemes that included initiatives for environmental protection.