In a period of growing urbanization and growing concern for environmental resilience, any design framework for sustainable cities includes a strategy on open spaces. One of the most common strategies is the metropolitan park system. It interconnects open spaces to form a green infrastructure on a metropolitan scale. This strategy embodies the landscape inversion approach where ecological and landscape processes, through open spaces, shape urban development. Meanwhile, urban wastelands are gaining increasing attention. When transformed into informal open spaces, they are increasingly recognized for their ecological and social values. However, wastelands keep being highly coveted for urbanization. In addition, little has been said about the important role of wastelands in the formation of green continuities on a metropolitan scale. This chapter foregrounds the question of wastelands by focusing on the intensity of the urban pressure on three semi-natural spaces in Brussels compared with their location within or without five green network projects for Brussels. The case of semi-natural spaces in Brussels is analyzed, as these spaces were previous urban wastelands that have been progressively transformed into informal open spaces. The analysis shows that the more a site is integrated into a park system, the lower is the expectation to build on it, and vice versa. It is important to specify that the integration of a site into a park system does not involve the loss of its informal characteristics. Additionally, case studies highlight the key role of wastelands in developing cross-boundary strategies and linking the core city with its hinterland.