We collect data on the size distribution of all U.S. corporations for 100 years. We document that corporate concentration (e.g., asset share or sales share of top businesses) has increased persistently over the past century. Rising concentration was stronger in manufacturing and mining before the 1970s, and stronger in services, retail, and wholesale after the 1970s. Furthermore, rising concentration in an industry aligns with greater technological intensity and more fixed costs. Industries with higher increases in concentration also exhibit higher output growth. Among the leading hypotheses for rising concentration, stronger economies of scale appear consistent with the long-run trends.