Preschoolers’ prosocial and aggressive behaviors were explored longitudinally, with a focus on the inferred underlying motives of these behaviors. Forty-four children (initially 22–40 months) were observed in naturalistic interactions with peers, during a two-month period for each of three consecutive years. Three categories of prosocial behavior (requested, altruistic, and non-altruistic) and three categories of aggressive behavior (reactive, proactive instrumental, and proactive hostile aggression) were explored for: (a) internal consistency; (b) developmental changes; (c) individual stability; (d) gender differences; and (e) interrelations. Internal consistency was moderately high for aggression and low for prosocial behavior. All types of prosocial behavior were enacted with increasing frequency as children grew older, whereas no developmental changes were revealed for the enactment of aggressive behavior. Individual stability was found for aggression and for prosocial altruistic behavior. A single gender difference was found: Girls outperformed boys on altruistic behavior at the end of the preschool period. Patterns of intercorrelations indicated that (a) prosocial requested behavior was unrelated to aggression; (b) prosocial altruistic behavior was negatively related to aggression, in particular to proactive hostile aggression; (c) prosocial non-altruistic behavior was sometimes positively related to aggression. The theoretical significance of focusing on underlying motives rather than on behavioral forms is discussed.