Teacher quality is widely believed to be important for education, despite little ev-idence that teachers ’ credentials matter for student achievement. To accurately measure variation in achievement due to teachers ’ characteristics–both observ-able and unobservable–it is essential to identify teacher fixed effects. Unlike previous studies, I use panel data to estimate teacher fixed effects while control-ling for fixed student characteristics and classroom specific variables. I find large and statistically significant differences among teachers: a one standard deviation increase in teacher quality raises reading and math test scores by approximately.20 and.24 standard deviations, respectively, on a nationally standardized scale. In addition, teaching experience has statistically significant positive effects on reading test scores, controlling for fixed teacher quality.