Sharp declines in the concentration of declared wealth occurred across Europe and the US during the 20th century. But the rich may have been hiding much of their wealth. This column introduces a new method to measure this hidden wealth, in any form. It finds that between 1920 and 1992, English elites concealed 20-32% of their wealth. Accounting for hidden wealth eliminates one-third of the observed decline of top 10% wealth share over the past century.