In the context of sustainable urban development, methodologies under the banner of sustainability assessment have the potential to contribute to the systematic operationalisation of sustainability as a decision-making strategy. To adequately capture the systemic complexities involved, sustainability assessments used in urban governance should be based on a comprehensive understanding of the urban systems in question. The aim of this presentation is to discuss a number of common metaphors used to describe cities, and to use the characteristics and contrasts between the metaphors as a point of departure for a critical reflection on the strengths and weaknesses of different assessment approaches to urban systems.