Three models regarding the relation between maternal (in)sensitivity, negative discipline, and child aggression were examined in a sample of 117 mother-child pairs with high scores on child externalizing behavior: (1) Sensitivity and discipline are uniquely related to child aggression (the additive model); (2) the relation between discipline and aggression is moderated by maternal sensitivity (the moderating model); (3) the relation between sensitivity and aggression is mediated by maternal discipline (the mediating model). Parenting and child aggression were observed when the children were on average 26.71 months old (range of 13.58 to 41.91 months) and again one year later. Results supported the moderating model. More negative discipline was related to more child aggression one year later, but only when mothers were insensitive. This finding supports the idea that the affective context is important for the impact of negative discipline on child development.