Behaviourism is a psychological theory that focuses on observable behaviors and reactions to stimuli rather than internal mental states. Some key behaviourist theorists include Ivan Pavlov, who discovered classical conditioning; Edward Thorndike, who introduced the law of effect and studied animal learning through puzzle boxes; John Watson, who promoted objective experimental methods and conducted controversial studies on humans; and B.F. Skinner, who developed operant conditioning and invented the Skinner box. Behaviourism views learning as occurring through conditioning processes involving stimuli, responses, and reinforcement or punishment.