Behavior of organism is controlled by not all elements of its environment, but a small part of it. In the paradigm of operant conditioning, such selectivity has been called "attention" or selective stimulus control". The authors critically reviewed studies about this phenomenon. Four factors, which determine the phenomenon, can be classfied as follows. 1) innate factor or factor of salience, 2) selective association between stimulus and reinforcer, 3) predictability of reinforcement, 4) history of stimulus control. Two different theories about attention have been proposed : i. e., selective attention theory and general attentiveness theory. The former claims that attention to one dimension reduces attention to other dimensions and the latter claims that attention to one dimension increases general attitude to attend to other dimensions. Recent experiments seem to support the general attentiveness theory.