The theory of classical conditioning was developed by the Russian physiologist Ivan Pavlov. His work began in the early 20th century with studies on the digestive processes of dogs, where he observed that dogs would salivate not just at the sight of food, but also in response to stimuli associated with food. Pavlov's findings led to the formulation of the concept of conditioning, where a neutral stimulus could elicit a conditioned response when paired repeatedly with an unconditioned stimulus. This groundbreaking theory has had significant implications in psychology, education, and behavioral science, influencing how we understand learning and behavior modification.