This study investigated the prospective links between sibling aggression and the development of externalizing problems using a multilevel modeling approach with a genetically sensitive design. The sample consisted of 780 adolescents (390 sibling pairs) who participated in 2 waves of the Nonshared Environment in Adolescent Development project. Sibling pairs with varying degree of genetic relatedness, including monozygotic twins, dizygotic twins, full siblings, half siblings, and genetically unrelated siblings, were included. The results showed that sibling aggression at Time 1 was significantly associated with the focal child's externalizing problems at Time 2 after accounting for the intraclass correlations between siblings. Sibling aggression remained significant in predicting subsequent externalizing problems even after controlling for the levels of preexisting externalizing problems and mothers' punitive parenting. This pattern of results was fairly robust across models with different informants. The findings provide converging evidence for the unique contribution of sibling aggression in understanding changes in externalizing problems during adolescence. (Contains 2 footnotes and 4 tables.)