Over the past quarter century, labor’s share of income in the United States has trended downwards, reaching its lowest level in the postwar period after the Great Recession. Detailed examination of the magnitude, determinants and implications of this decline delivers five conclusions. First, around one third of the decline in the published labor share is an artifact of a progressive understatement of the labor income of the self-employed underlying the headline measure. Second, movements in labor’s share are not a feature solely 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 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, institutional explanations based on the decline in unionization also receive weak support. Finally, we provide evidence that highlights the 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.