The RE-source research project is mapping urban residual streams in order to then use them as a source for circular thinking, doing and learning. RE-source analyzes and provides insight into the systems, locations, products and materials needed to do so. At the same time, it designs strategies to turn these residual streams into a resource from which to draw again and again. Through the City of Rotterdam's network, RE-source seeks to gain insight into the process of designing, managing and maintaining outdoor space and its material flows. RE-source investigates which phases are distinguished, what the routes used are and which human and non-human actors - such as locations, citizens and professionals involved, tools, vehicles, distances - play a role in it.