Teachers differ greatly in how much they teach their students, but little is known about which teacher attributes account for this. We estimate the causal effect of teacher subject knowledge on student achievement using within-teacher within-student variation, exploiting a unique Peruvian 6th-grade dataset that tested both students and their teachers in two subjects. Observing teachers teaching both subjects in one-classroom-per-grade schools, we circumvent omitted-variable and selection biases using a correlated random effects model that identifies from differences between the two subjects. After measurement-error correction, one standard deviation in subject-specific teacher achievement increases student achievement by about 9% of a standard deviation in math. Effects in reading are significantly smaller and mostly not significantly different from zero. Effects also depend on the teacher-student match in ability and gender.