The present study investigates the effects of teachers (background qualifications, attitudes and beliefs, and instructional practices) on student achievement in mathematics, reading, and spelling in 1st grade. Its theoretical framework and methodology are based on recent work by Palardy and Rumberger (2008). Data from the SiBO Project, a longitudinal study in Flemish primary education, were analyzed for the present study using 2-level regression analysis. The results showed that teacher background had the largest effect on mathematics achievement, whereas instructional practices had the largest effects on both reading and spelling achievement. Moreover, the size of teacher effects depended upon the specific learning domain: Larger teacher effects were found for mathematics than for spelling and reading. Overall, the results suggest that teachers had a modest to strong effect on student achievement in first grade.