Stocks of products in use are the pivotal engines that drive anthropogenic metal cycles: They support the lives of people by providing services to them; they are sources for future secondary resources (scrap); and demand for in-use stocks generates demand for metals. Despite their great importance and their impacts on other parts of the metal cycles and the environment, the study of in-use stocks has heretofore been widely neglected. Here we investigate anthropogenic and geogenic iron stocks in the United States (U.S.) by analyzing the iron cycle over the period 1900 -2004. Our results show the following. (i) Over the last century, the U.S. iron stock in use increased to 3,200 Tg (million metric tons), which is the same order of magnitude as the remaining U.S. iron stock in identified ores. On a global scale, anthropogenic iron stocks are less significant compared with natural ores, but their relative importance is increasing. (ii) With a perfect recycling system, the U.S. could substitute scrap utilization for domestic mining. (iii) The per-capita in-use iron stock reached saturation at 11-12 metric tons in Ϸ1980. This last finding, if applicable to other economies as well, could allow a significant improvement of long-term forecasting of steel demand and scrap availability in emerging market economies and therefore has major implications for resource sustainability, recycling technology, and industrial and governmental policy.dematerialization ͉ material flow analysis ͉ resource management ͉ secondary resource exploration ͉ ferrous scrap recycling I n 1969, the urbanist Jane Jacobs referred to cities as ''the mines of the future'' (1). Her perspective was that resources that have been mined, processed, and fabricated into products constituted a material stock that could eventually supplant in-ground ore. Almost 4 decades later, and with still limited knowledge of these urban mines, we recognize significant differences between urban and traditional mines. First, whereas mineral ores change very slowly over time, anthropogenic stocks change rapidly and therefore require better monitoring. Second, mining production of mineral ores can readily be adjusted to changes in demand, provided that necessary reserves, capital, and labor are available, whereas urban mining faces physical limitations because it is restricted to products in use becoming obsolete. Third, the material in urban mines is generally of higher quality than mineral ores (2), because already processed and purified material often requires less energy and technology to re-employ. Fourth, there is extensive knowledge about the size and chemical and physical properties of geological ores, but there is very little understanding of anthropogenic material stocks and their dynamics.The lack of knowledge about in-use stocks not only limits our insights into future resources, but it also confines our understanding of entire mineral cycles. Comprehending in-use stocks is therefore essential for measuring and improving overall resource utilization.The study of a...