Aggression and cooperation, which represent two critical features in the child's social domain, have one element in common: they both emerge from children's strong developmental push to initiate and maintain relationships with other children. Aggression is defined as any intentional behavior that results in physical or mental injury to any person or animal, or in damage to property. Aggressive actions can be accidental, instrumental, or hostile. Aggression should not be confused with assertion, which is behavior through which children maintain and defend their rights. Cooperation is defined as any activity that involves the willing interdependence of two or more children. Cooperation should be distinguished from compliance, which represents obedience to authority rather than intentional cooperation. Techniques adults can use for reducing children's aggression include helping children to verbalize their feelings, develop problem-solving approaches to conflicts, and notice the consequences of their aggressive actions. To foster children's cooperation, adults can acknowledge children's efforts to initiate appropriate social interactions, affirm helping behaviors, communicate positive regard, and support children's struggles to resolve interpersonal conflicts. In various ways, early childhood educators can support the emergence of children's positive interpersonal strategies, and early childhood programs can help parents understand the range of children's emerging social repertoires. (BC)