AbstractAnimal experiments in which the opportunity to aggress was positively reinforcing are surveyed, with attention paid to field and laboratory reports on play and play fighting. A distinction is drawn between ways in which aggressive behavior may be elicited and factors which increase or decrease the probability that aggression will be emitted. Since aggression may be emitted and may also be reinforcing, the possibility that aggressive behavior may be self‐reinforcing is mentioned. In addition to positive reinforcement, consequences of aggressive behavior are presented. These include punishment, removal of a noxious stimulus, feeding, arousal, and disinhibitio