Latent growth curve modeling was used to study the co-development of internalizing and externalizing problems in a sample of 2844 Korean fourth graders followed over four years. The project integrated two major theoretical viewpoints positing developmental mechanism: directional model and common vulnerability model. Findings suggest that (a) boys and girls follow different developmental trajectories in both domains in early adolescence; (b) bidirectional progression from initial levels of each domain to the developmental pattern of the other domain emerged among boys, while only unidirectional progression from externalizing to internalizing problem emerged among girls; and (c) all risk factors are not equally risky across domain and gender; parental violence was a common cross-domain risk factor for boys, whereas affiliation with delinquent friends was a common cross-domain risk factor for girls. Implications for future research and intervention were discussed. (Contains 5 tables and 6 figures.)