Cities are complex socio-ecosystems in constant transformation. The intensity and specificity of the anthropogenic impact of urban areas on resources (water, soil, air, vegetation, living organisms) and biogeochemical cycles have led to the introduction of the concept of the Urban Critical Zone. The Urban Critical Zone is at the heart of local environmental and economic issues (management of land use, water, pollution and disturbances) and more global issues (climate, energy or erosion of biodiversity). An analysis of the scientific and societal challenges to face highlights that efforts have to be particularly focused on the understanding of mechanisms of urban development and their impact on the urban ecosystem in an integrative way, in order to optimise measures for mitigating and adapting to global change. The Service National d’Observation (SNO) Observil was created in 2020 by the CNRS. It relies on a network of 11 observatories and 25 research teams spread out all over France. The Observil network is focused on the physical, biological and geochemical functioning of urban environments, submitted to a strong anthropic pressure in a context of global change. It focuses in particular on the interactions between the built environment (buildings, roads, surface or in-ground urban developments) and the other components of the urban ecosystem. The way in which urban space is used, the way it is managed and the intensity of human activities determine the anthropogenic forcings that are added to the natural forcings. This need for knowledge and understanding of urban environments is a concern for local authorities, whose planning choices are linked to the environmental quality of the urban space they manage.