<p>Resources are essential for humanity's well-being and development. At the same time, resources lay at the heart of many environmental problems. A sustainable resource use facilitates development but reduces environmental problems. This apparent contradiction can be solved by moving toward a circular economy: keeping resources, once extracted, in use for as long as possible. The services fulfilled by the in-use stocks of resources can thus be maintained or even increased, while the inflow of virgin materials into the stocks can be reduced. In this view, society's material basis shifts from geological mines to urban mines: materials locked in products and infrastructures presently used by societies. Although very important, the information on such urban mines is scarce. In this paper, we present the results of an inventory of the urban mine of the Netherlands. We assessed 86 categories of applications, both products and infrastructures, and 53 different materials. The Dutch urban mine adds up to 447 tonnes of materials per inhabitant, mostly construction materials. The inventory can be regarded as a pilot project. It indicates an approach for assessing the urban mine and how to standardize such an effort. It can also be seen as a first estimate of what we collectively have to work with to realize a circular economy. The overall conclusion is that the urban mine is of considerable size and could be a major source of materials but it will take a considerable time and some major changes in the organization to realize that.</p>