When experimenters require their subjects to perform some readily recorded response to gain access to discriminative stimuli but do not permit this behavior to alter the schedule of reinforcement, the response is classified, by analogy, as an “observing” response. Observing responses have been used not only to analyze discrimination learning but also to substantiate the concept of conditioned reinforcement and to measure the reinforcing effect of stimuli serving other behavioral functions. A controversy, however, centers around the puzzling question of how observing can be sustained when the resulting stimuli are not associated with any increase in the frequency of primary reinforcement. Two possible answers have been advanced: (a) that differential preparatory responses to these stimuli as conditional stimuli make both the receipt and the nonreceipt of unconditional stimuli more reinforcing; and (b) that information concerning biologically significant events is inherently reinforcing. It appears, however, that the stimulus associated with the less desirable outcome is not reinforcing. The maintenance of observing can be reconciled with the traditional theory that the acquisition of reinforcing properties proceeds according to the same rules as those for Pavlovian conditioning if it is recognized that the subject is selective in what it observes and procures a greater than proportionate exposure to the stimulus associated with the more desirable outcome. As a result of this selection, the overall frequency of primary reinforcement increases in the presence of the observed stimuli and declines in the presence of the nondifferential stimuli that prevail when the subject is not observing.