One of the first papers to ever estimate teacher effects at the secondary school level, this groundbreaking work presents evidence that teacher credentials affect secondary school student success in systematic ways and to a significant, policy-relevant extent. We use data on statewide end-of-course tests in North Carolina to examine the relationship between teacher credentials and student achievement at the high school level. We find compelling evidence that teacher credentials affect student achievement in systematic ways and that the magnitudes are large enough to be policy relevant. As a result, the uneven distribution of teacher credentials by race and socio-economic status of high school students- a pattern we also document- contributes to achievement gaps in high school.