The research project RE-source maps out urban residual flows, in order to then use them as a source for circular thinking, doing and learning. RE-source analyses and provides insight into the structure of the systems, locations, products and materials that are needed for this. At the same time it designs strategies for transforming these residual flows into a resource which can be used again and again. Through the network of the Rotterdam city council, RE-source tries to gain insight into the method of designing, managing and maintaining the outside space and associated material flows. RE-source researches which phases are distinguished, which routes are used and which human and non-human factors – such as locations, involved citizens and professionals, tools, vehicles, distances – play a role in this. This is collaboration at its highest level, on the very current topic of circularity.