Ivan Pavlov looms large in the behaviorist pantheon. His theory of classical conditioning confirms the crucial part that antecedents play in learned behavior. Unconditioned stimuli produce unconditioned or natural responses, such as Pavlov's dogs salivating at the smell of meat powder. By combining a known stimulus and a natural response with a neutral stimulus such as a ringing bell, Pavlov found that dogs would begin to salivate upon hearing the bell alone, a conditioned stimulus producing a conditioned response. Since such classical conditioning creates both behavioral and emotional responses, it is used to explain and treat phobias, anxieties and aberrant behavior.

The key classical conditioning phenomena of stimulus generalization, extinction, and counter conditioning may be explained briefly by the following:

1. Stimulus generalization - a conditioned stimulus is repeated along with another like stimulus until the latter alone produces the response.
2. Extinction - a conditioned response fades over time as a conditioned stimulus is repeated without the unconditioned, natural stimulus.
3. Counter conditioning - a conditioned stimulus is coupled with another stimulus to evoke a response contrary to that produced by the original stimulus.