This paper investigates achievement gaps between low and high socioeconomic students in 19 high-income countries. On average, math scores of students with indicators of high socioeconomic status (SES) are over one standard deviation above those with low SES indicators. The paper estimates the extent to which these achievement gaps can be attributed to differences in classroom- and school-level resources available to students from different SES backgrounds. In some countries, achievement gaps can be largely explained by differences in the characteristics of schools attended. However, in many other countries, the gap appears more closely related to differences in the characteristics of the students. The results point to the importance of institutional difference among countries in explaining international differences in the quality of education received by different groups within a nation. (Contains 4 tables, 2 figures and 13 notes.)