Urban areas cover 2% of the Earth's land surface, host more than 50% of global population and are estimated to account for around 75% of CO2 emissions from global energy use. In order to mitigate existing and future direct and indirect environmental pressures resulting from urban resource use, it is necessary to investigate and better understand resource and pollution flows associated with urban systems. Current urban environmental assessment methodologies enable the quantification of resource use and pollution emissions flows entering, becoming stocked and exiting urban areas. While these methodologies enable to estimate the environmental effect of cities, they often consider urban areas as being static and homogeneous systems. This partial and simplistic representation shadows the complex spatio-temporal interrelationships between the local context and its associated local and global environmental pressures. This characterisation of urban systems is a significant limitation, not only for the urban environmental assessments, but also for the identification of their drivers as it may lead to inadequate urban environmental policies. To overcome this limitation and effectively reduce glocal urban environmental pressures, it is necessary to better understand the complex functioning of cities and identify their drivers. This research developed a comprehensive urban environmental assessment framework that helps to better explicit and understand the complex relationship between an urban system and its environmental profile in a systemic and systematic way. This framework was applied to the case study of Brussels Capital Region (BCR). Results from the application of this framework show that urban systems are neither static nor homogeneous. In fact, different relationships between the urban and metabolic profiles appear when considering different spatial scales and temporal intervals as well as different urban and metabolic metrics. The establishment of BCR's urban profile showed that components that shape the urban system evolve in an organic way over time. Moreover, the spatial expression of an urban system portrays its heterogeneous aspect and how different metrics of the same urban indicator can reveal distinct facets and challenges for an urban area or a neighbourhood. Finally, it was demonstrated that the relationship between urban indicators is different for each spatial scale and therefore knowledge from one spatial scale is not necessarily transferable from one scale to another. The establishment and analysis of BCR's metabolic profile also underlined the complex functioning of cities as each flow has a different temporal evolution and spatial expression. Due to the multifaceted and intertwined aspect of metabolic flows it becomes clear that no single parameter enables to explain or predict their behaviour. This leads to the conclusion that a great number of questions still need to be considered, understood and answered before effectively and coherently reducing environmental pressures from cities. The developed framework proposes a number of concrete steps that enable existing and new cities to better understand their metabolic functioning and ultimately transition towards less environmentally harmful futures.