The punishment procedure is one in which an aversive stimulus is contingent upon the occurrence of a response. Various theories of the mechanism through which punishment exerts its influence on behavior emphasize the unconditioned fear response, the unconditioned skeletal response, the escape response, the similarity between the conditions of punishment and the conditions of training, the correlation of response and punishment, and the possible sources of reinforcement for nonresponse. The major problem of this paper was to determine whether any of the proposed mechanisms, or a combination of them, are sufficient to account for the varied effects of punishment on behavior. A systematic examination of the data led to the conclusions that if an aversive stimulus is contingent upon a response there will be greater suppression (or less facilitation) of the response than if the aversive stimulus is not contingent upon the response, but that the aversive stimulus, itself, may result in response facilitation under some conditions and response suppression in others.