What are wastes? In our material world, we have to manage a massive quantity of product and by-product, from organic and inorganic systems. This global movement, determined mostly by the expansion of industrial activities and mass consumption, is characterised by a significant growth of the environmental human footprint. Each urban civilisation implements strategies for the collection, removal and elimination of their flows (effluents and solid waste). 'Recycling' a city implies considering three major dimensions that characterise our urban metabolism in modernity: the dialectic of a city grappling with its (material and symbolic) history, between ruins and remnants all while dealing with the imperatives of 'rebuilding the city'; environmental crises that lead to more cost-efficient resource management and environmental protection, according to a still fuzzy model of urban metabolism yet to be designed; and the emergence of health risk factors outside the 'waste' category in the form of imperceptible micro pollutants that make up urban environments.