Using nationally representative data on 12,462 kindergarten children, this report examines the link between behavioral engagement and math achievement growth during kindergarten. Multilevel models show that students with higher individual engagement tend to experience larger math achievement growth over kindergarten, that classroom engagement makes a difference in how much achievement growth students experience over kindergarten, and that students with higher individual engagement benefit more from being in highly-engaged classrooms than children with lower individual engagement. Students with higher math test scores at kindergarten entry also benefit more from highly-engaged classrooms than children with lower prior math scores. Results from this study provide new evidence that behavioral engagement affects students' achievement growth on multiple levels, with the individual, the classroom environment, and the interaction of the individual and classroom environment all relating to math outcomes. Evidence reported from this study is central to reducing educational inequalities.