This study attempts to explain the effects of ethnic and linguistic diversity on educational attainment. We argue that cross-section differences in ethnic and linguistic fractionalization can explain a substantial part of the cross-country differences in educational attainment levels. Using a data on 86 countries, we uncover new evidence on the relationship between fractionalization and educational attainment. We find that fractionalization lower educational attainment. This finding is consistent across various measures of educational attainment, and is robust to several sensitivity checks. We explore several potential mechanisms which could explain the observed negative effects of ethnic and linguistic diversity including ethnic diversity’s effect on social capital, discrimination, public goods, conflicts, and institutional quality, among others.