Ever since Pavlov (1927) extended the concept of the reflex to the study of conditioned reflexes in classical conditioning, behavioral scientists have sought to analyze the control exerted by stimuli over behavior. Many stimulus classes - that is, sets of similar or related stimuli - exert control over behavior in the sense that certain responses become much more probable (or much less probable) when a member of the stimulus class is present. As with the other phenomena that we have introduced in earlier chapters, such stimulus control is a characteristic not only of human behavior but also that of many other animal species. Consequently, we will be able to draw on the results of experimental work with various species in order to illustrate its features. However, we will begin by identifying some defining characteristics of stimuli and stimulus classes.