The present study proposes to investigate the development of aggressive and depressive problems during adolescence using developmental psychopathology as a conceptual framework. Developmental psychopathology is concerned with individual differences in the predictors, course, and outcomes of adaptive and maladaptive developmental processes. Three objectives are proposed: (1) to identify groups of adolescents exhibiting differential levels of distinct or combined aggressive and depressive problems across time, (2) to investigate the relationship of risk and protective factors to aggressive and depressive problems, and (3) to identify differential outcomes of aggressive and depressive problems. These objectives will be tested in a sample of 700 European adolescents. To investigate co-occurrence during adolescence a longitudinal model will be employed across four consecutive years (grades 7 to 10). Data will be analyzed using the longitudinal person-oriented methodology General Growth Mixture Modeling (GGMM). This is the first study to investigate the co-occurrence between aggressive and depressive problems using GGMM, and the study is expected to provide evidence of how and why aggressive and depressive problems, two symptoms from different diagnostic classes representing separate forms of psychopathology, co-occur in children. Furthermore, findings of the current study can provide evidence that may lead to effective interventions and treatments. Moreover, using the template provided by the proposed study on how to investigate co-occurrence, future studies conducted in the EU can inform the world about co-occurring disorders and therefore enhancing EU scientific excellence. The Marie Curie International Reintegration Grant will enhance the career development of the researcher, strengthen his collaborations with researchers in the United States, and allow the researcher to offer his expertise at community level and also enhance EU competitiveness.