The United States is an outlier on many measures of inequality. When compared to other well-off countries, it has unusually high levels of income inequality, unusually high levels of wealth inequality, and unusually high levels of poverty. The purpose of this article is, in part, to ask whether the “income achievement gap”—the test score gap between children from high- and lowincome families—is also unusually high in the U.S. This gap is important because it reflects (a) the extent to which students experience different socioeconomic conditions in their early childhood and different schooling conditions once they reach school age, and (b) the extent to which these socioeconomic and schooling context differences lead to different educational outcomes (test scores, in this case). It may accordingly be understood as an early (albeit obviously imperfect) measure of the extent to which opportunities are unequal. Although a main purpose of this article is simply to establish how the U.S. stacks up against its peer countries on this key measure of unequal opportunity, our follow-up objective is to cast some light on the sources of international differences in this measure. We examine, in particular, whether income inequality is an important source of the achievement gap. The evidence from the U.S. is at least suggestive of an “income inequality” effect: In the 1980s and 1990s, as income inequality in the U.S. grew sharply, so too did the academic achievement gap by family income. That family income and family socioeconomic status (SES) are related to children’s academic achievement is not surprising; that this relationship grew so rapidly in the U.S. in the last several decades, however, is rather surprising. The U.S. trends suggest that some of this growth may have been the result of rising income inequality.