Classical conditioning occurs when a stimulus that elicits a response is paired with another stimulus that initially does not elicit a response on its own. Over time, the second stimulus causes a similar response because we associate it with the first stimulus. Ivan Pavlov, a Russian physiologist who conducted research on digestion in animals, first demonstrated this phenomenon in dogs.

He paired a neutral stimulus (a bell) with a stimulus known to cause a salivation response in dogs. The powder was an unconditioned stimulus (UCS) because it was naturally capable of causing the response. Over time, the bell became a conditioned stimulus (CS). The bell did not initially cause salivation but the dogs learned to associate the bell with the meat powder and began to salivate at the sound of the bell only. The drooling of these canine consumers because of a sound was a conditioned response (CR).

Conditioning effects are more likely to occur after the conditioned (CS) and unconditioned (UCS) stimuli have been paired a number of times. This effect is known as repetition. Stimuli similar to a CS may evoke similar responses. This is known as stimulus generalization. Conditions may also weaken over time especially when a UCS does not follow a stimulus similar to a CS. This is called stimulus discrimination