This study uses Early Childhood Longitudinal Study data to investigate the importance of three general aspects of teacher effects--teacher background qualifications, attitudes, and instructional practices--to reading and math achievement gains in first grade. The results indicate that compared with instructional practices, background qualifications have less robust associations with achievement gains. These findings suggest that the No Child Left Behind Act's "highly qualified teacher" provision, which screens teachers on the basis of their background qualifications, is insufficient for ensuring that classrooms are led by teachers who are effective in raising student achievement. To meet that objective, educational policy needs to be directed toward improving aspects of teaching, such as instructional practices and teacher attitudes. (Contains 4 tables, 1 figure and 33 notes.)