This longitudinal study examined different explanations for negative associations between aggression and academic achievement using data collected from 403 children from low-income families followed from kindergarten or first grade (ages 6 and 7 years) through fifth grade (ages 10–11 years). Most results of growth curve analyses examining change over time and path analyses examining associations among the variables within grades were consistent with the hypothesis that the effect of aggression on achievement was partially mediated by the conflictual relationships relatively more aggressive children tended to develop with their teachers and concomitant reductions in engagement in academic tasks. The evidence suggested, however, that the relationship between aggression and achievement is complex and reciprocal. Gender differences were also observed.