The 2003 Trends in International Mathematics and Science Study data from 46 countries showed that, although the national level of teacher quality in the United States was similar to the international average, the opportunity gap in students' access to qualified teachers between students of high and low socioeconomic status (SES) was among the largest in the world. Cross-national analyses revealed that the countries with better teacher quality produced higher mathematics achievement. However, larger opportunity gaps in access to qualified teachers did not predict larger achievement gaps between high-SES and low-SES students cross-nationally. These analyses provide empirical, cross-national evidence of the importance of investing in teacher quality for improving national achievement. National policies and practices related to improving teacher quality appear to be a promising area for future research to identify how other countries have achieved both excellence and equity in student achievement. (Contains 2 figures, 4 tables and 9 notes.)