Using time-series data from the US since 1950 and from 53 countries around the world in 2006, this chapter documents a strong negative relation between an economy’s employment concentration (that is, the proportion of the labor force employed by the largest 10, 25, or 50 firms) and its level of income inequality. Within the US, we find that trends in the relative size of the largest employers (up in the 1960s and 1970s, down in the 1980s and 1990s, up in the 2000s) are directly linked to changes in inequality, and that corporate size is a proximal cause of the extravagant increase in social inequality over the past generation. We conclude that organization theory can provide a distinctive contribution to understanding societal outcomes.