Abstract : 'Open space' is a term used to qualify all unbuilt spaces with rural and natural characteristics, located within large urban areas. Although this expression is being abandoned in North America, it is getting growing interest in Europe. This tendency comes from difficulties in identifying the great diversity of unbuilt spaces, but also from the fact that words used to characterize such spaces are often not well adapted. Concepts of landscape, countryside, nature still give importance to the difference between urban spaces or urban knowledge and non-urban ones. The use of many oxymorons (city-nature, city-landscape, city-countryside) reveals reluctance to accept the reality of general urbanization in the relationship societies have with all the spaces, whatever their shape. The present work questions the reasons leading to find another word to qualify those spaces and the process that allows them to emerge. We identify three fields feeding this process: city and urbanistic thinking, society/city-nature relationship, public action. The open space exists through the existence of cities. It comes from shapes that created the evolution of city-countryside relationships, and from the way city and people making the city look at unbuilt spaces (part 1). The evolution of society/nature relationships in a contemporary world that finds its limits imposes rethinking the relationship between city and nature. These terms have been opposed for a long time but are now associated. Spatial strategic planning participates to this new relationship and to the visibility of open space (part 2). This visibility is substantiated through public action creating territorial projects, which allow urban planning management to be renewed (part 3). In view of the great diversity of situations and processes related to open spaces, whether close to or far away from cities, 'open space' is a useful portmanteau word because it has wide enough sense to integrate all the diversity without qualifying it at first. It is thus a useful tool to understand the way territorial agents (public or private) conceive and appropriate those spaces. It allows one to observe the dynamics operating in urban peripheries, as well as the emerging urbanity forms in which unbuilt spaces and practices of nature tend to assert themselves. This is the hypothesis that this works defends.