This introduction to the special issue Flux 2017/2 (N° 108) questions the growing political injunction to insert urban objects, waste and material flows into the circular economy. The collected papers focus on the materiality of the waste and objects that populate and constitute the city. In addition, the articles analyse how and with which consequences such materials are put in motion and circulate, with special emphasis on the regulations and conflicts that they bring about. Most of the articles present cases studies from the urban area of Lyon, France. This is more by incident than by design. However, this geographical framing highlights the importance of placing and scaling material issues adequately. Three transversal themes are identified. First, all texts show how the invention of new material and waste resources in cities is generative of new flows and spatial circulations. Second, the regulation of such circulations generally rests on new forms of governance, themselves staked on the identification and empowerment of new actors that come between the producers and the managers of the matter that circulates. Lastly, it appears that these new circuits are mostly to be found in a space defined by geographic and relational proximities. Altogether, the contributions of this special issue help nuance and rematerialize the political calls to circularize the urban economy.