On the basis of a new model of motivation, we examined the effects of 3 dimensions of teacher (n= 14) behavior (involvement, structure, and autonomy support) on 144 children's (Grades 3–5) behavioral and emotional engagement across a school year. Correlational and path analyses revealed that teacher involvement was central to children's experiences in the classroom and that teacher provision of both autonomy support and optimal structure predicted children's motivation across the school year. Reciprocal effects of student motivation on teacher behavior were also found. Students who showed higher initial behavioral engagement received subsequently more of all 3 teacher behaviors. These findings suggest that students who are behaviorally disengaged receive teacher responses that should further undermine their motivation. The importance of the student–teacher relationship, especially interpersonal involvement, in optimizing student motivation is highlighted.