Following leads from differential emotions theory and empirical research, we evaluated an index of emotion knowledge as a long-term predictor of positive and negative social behavior and academic competence in a sample of children from economically disadvantaged families (N = 72). The index of emotion knowledge represents the child's ability to recognize and label emotion expressions. We administered control and predictor measures when the children were 5 years old and obtained criterion data at age 9. After controlling for verbal ability and temperament, our index of emotion knowledge predicted aggregate indices of positive and negative social behavior and academic competence. Path analysis showed that emotion knowledge mediated the effect of verbal ability on academic competence. We argue that the ability to detect and label emotion cues facilitates positive social interactions and that a deficit in this ability contributes to behavioral and learning problems. Our findings have implications for primary prevention.