Over the past quarter century, labor’s share of income in the United States has trended downward, reaching its lowest level in the postwar period after the Great Recession. A detailed examination of the magnitude, determinants, and implications of this decline delivers five conclusions. First, about a third of the decline in the published labor share appears to be an artifact of statistical procedures used to impute the labor income of the self-employed that underlies the headline measure. Second, movements in labor’s share are not solely a feature of recent U.S. history: The relative stability of the aggregate labor share prior to the 1980s in fact veiled substantial, though offsetting, movements in labor shares within industries. By contrast, the recent decline has been dominated by the trade and manufacturing sectors. Third, U.S. data provide limited support for neoclassical explanations based on the substitution of capital for (unskilled) labor to exploit technical change embodied in new capital goods. Fourth, prima facie evidence for institutional explanations based on the decline in unionization is inconclusive. Finally, our analysis identifies offshoring of the labor-intensive component of the U.S. supply chain as a leading potential explanation of the decline in the U.S. labor share over the past 25 years.