The rise of mass-market merchandisers is one of the most important aspects of the modern American economy. The retail and wholesale trade sectors were especially important parts of productivity growth in the late 1990s, and by at least one estimate, the effect of modern discount retailers had an effect on the American economy that was similar in magnitude to the effect of railroads on the American economy in the nineteenth century. Retail establishments have gotten larger and more concentrated; the mass-market merchandisers of the later part of the twentieth century carried on a trend toward consolidation of the retail sector into national chains operating large stores that started before they were founded. Wal-Mart is the world’s most important and controversial retailer: the evidence on Wal-Mart’s effects on retail employment suggests either mild positive or mild negative effects, but Wal-Mart’s biggest impact has been on prices. Evidence suggests that Wal-Mart’s price effect leads to increases in real income.