We investigated the effects of competitive and cooperative games on aggressive and cooperative behaviors of 70 children (4 to 5 years old) from four dasses in three preschools. The experimental design induded both multiple baseline and reversal components. Behaviors were measured during game conditions and in subsequent free-play periods. Results showed that cooperative behavior increased and aggression decreased during cooperative games; conversely, competitive games were followed by increases in aggressive behavior and decreases in cooperative behavior. Similar effects were also found during free-play periods. r DESCRIPTORS: cooperative games, preschool children, aggressive behavior, competition, generalization There is general agreement on the importance of children's development of strong positive social skills. The ability to interact in positive, nonaggressive, collaborative ways with others is one of the most fundamental goals of development and provides a basis for success in friendships, marriage, and careers. Much ofthe learning ofthese behaviors is done through play (Bruner, 1975). In effect, play is the child's workshop, a place where rules, behaviors, and consequences are explored, changed, and learned.Games are a central aspect of children's play. Considerable research has focused on how games influence behavior. For example, Murphy, Hutchison, and Bailey (1983) found that the aggressive playground behaviors ofelementary school children were reduced when free (unstructured) play was replaced by organized games (rope jumping and foot races) and a time-out procedure.