The continued increase in the use of metals over the 20th century has led to the phenomenon of a substantial shift in metal stocks from the lithosphere to the anthroposphere. Such a shift raises social, economic, and environmental issues that cannot be addressed without quantifying the amount of stock of "metal capital" utilized by society. Estimation of the in-use stock of metals has occurred for at least 70 years, with over 70% of the publications occurring after the year 2000. Despite the long history, this is the first critical review to consolidate current findings, critique methods, and discuss future avenues of research. Only aluminum, copper, iron, lead, and zinc have been studied to any extent Nonetheless, it is clear that for the more-developed countries, the typical per capita in-use metal stock is between 10 and 15 t (mostly iron). Comparison of the per capita stocks in more-developed countries with those in less-developed countries suggests that if the total world population were to enjoy the same per capita metal stock levels as the more-developed countries, using a similar suite of technologies, the amount of global in-use metal stocks required would be 3-9 times those existing at present.