Although the majority of elementary school teachers cover all major subjects in self-contained classrooms, a growing number of teachers specialize in teaching fewer subjects to higher numbers of students. We use administrative data from Indiana to estimate the effect of teacher specialization on teacher and school effectiveness in elementary schools. We find that teacher specialization leads to lower teaching effectiveness in math and reading, and the negative effects are larger when teaching students who are more likely to experience obstacles in school. Moreover, we find no evidence that increasing the proportion of teacher specialists at the school level generates improvements in indicators of school quality. Our findings underscore the importance of fostering opportunities to develop stronger student--teacher relationships.