This chapter investigates the sources of variation in two core outcomes of interest to economists in the United States—the earnings distribution and mobility patterns—and presents a brief literature review and institutional background. It also briefly discusses the new database infrastructure, and then reports some basic statistics about the structure of wages within and between firms, as well as job mobility patterns. The underlying dynamics of earnings inequality are complex and are due to factors that cannot be measured in standard cross-sectional data. While there is substantial between-firm variation in earnings, the within-firm variation is very large in terms of both levels and changes of earnings over time. The earnings dynamics and turnover are, not surprisingly, connected with workers that change jobs having on average a positive increase in earnings.