Whereas Pavlovian conditioning involves a contingency relationship between two environmental stimuli (the CS and UCS), instrumental conditioning involves a contingency between some aspect of the organism’s behavior and some aspect of the environment, such that the organism’s behavior modifies its environment in some way. Whereas Pavlovian conditioning involves stimulus-stimulus (CS–UCS) contingencies, instrumental conditioning involves response-stimulus contingencies. Instrumental conditioning acts to alter the frequency of the response involved in the contingency relationship in a predictable way, depending on the nature of the consequence of that response.