The goal of this study was to examine the links between difficult temperament (i.e., negative emotionality) and harsh parental discipline during toddlerhood, and reactive and proactive aggression in kindergarten. These links were assessed on a longitudinal population-based study of 1516 boys and girls followed longitudinally from the age of 17 months through the age of 72 months. Two possible models were tested to examine the interplay between negative emotionality and harsh parenting in predicting later reactive aggression compared to proactive aggression. The first was an additive model where both aspects make unique contributions in predicting later reactive aggression. The second model was an interactive model where harsh parenting exacerbates the link between negative emotionality and reactive aggression. Results showed a specific contribution of negative emotionality to reactive aggression. The results relative to harsh parenting are more mixed but nonetheless in line with developmental models stressing different pathways to reactive and proactive aggression.