The aim of this study was to analyse the cognitive processes (prosocial moral reasoning, perspective taking) and emotional processes (empathic concern, emotional instability, state-trait anger) which interact in predicting aggressive behaviour and prosocial behaviour of adolescents who have committed a crime and those who have not, for the purpose of establishing the predictor variables in both groups. Participants were 440 adolescents, 220 of them young offenders residing in four youth detention centres in Valencia, in which they were serving court sentences (67.3% men and 32.7% women). The other 220 were enrolled in public and private schools within the metropolitan area of Valencia (65.9% men and 34.1% women). The two subsamples were equated in age (15-18 years) and sex, controlling the representation of social classes. Prosocial moral reasoning, empathy, emotional instability, state-trait anger, prosocial behaviour, and physical and verbal aggression were assessed. Hierarchical regression analyses show the differential weight of positive emotions (empathic concern) and negative emotions (emotional instability and anger) in relation to prosocial moral reasoning in predicting aggressive behaviour in adolescents, especially offenders. The results are discussed in terms of their implications for prevention and re-education oriented to social reinsertion of young offenders.