We introduce the construct of perceived containment, defined as children's beliefs about adults' capacity to impose firm limits and to prevail if there is a conflict in goals. We propose that children's containment beliefs represent an important but understudied factor in the development and maintenance of childhood aggression. Children's ratings on the Perceived Containment Questionnaire (PCQ) were inversely related to parent and teacher ratings of externalizing problems. Moreover, this relation was found to be independent of the quality of parental discipline. We also found evidence that perceived containment moderated the relation between overly harsh, inept discipline and children's externalizing behaviors: ineffective discipline was directly related to externalizing problems in children with relatively high PCQ scores but was unrelated to externalizing problems in children with relatively low PCQ scores. For the latter group of children, the affective quality of the mother–child relationship was a better predictor of problem behavior. These findings provide additional support for Kochanska's (1993) model of differential socialization and for Frick's (1998) assertions concerning meaningful subgroups of aggressive children.