Behavioral psychology was immediately preceded by the reflexology of Russian physiology and the associationism of Thorndike. Physiological reflexology received a sound foundation with the works of Sechenov and Bekhterev, but it was Pavlov who proposed a comprehensive theory of conditioning. Watson’s behavioral formulation defined stimulus and response elements as the substitute to rid psychology of residual mentalistic constructs. Watson’s contemporaries, Holt, Weiss, Hunter, and Lashley, soon restored to behaviorism critical psychological activities. The logical positivist movement expressed an operational spirit and insured the initial success of the behaviorist model. Behavioral psychology expanded beyond the original formulations of Pavlov and Watson. Contemporary reflexology in Russia and in nearby countries expanded to include a wide range of psychological and physiological problems, led by such eminent scientists as Vygotsky, Luria, Konorski, Asratyan, and Beritashvili. In the United States, behaviorism moved through several intellectual stages, through the contributions of Guthrie, Tolman, Hull, and Skinner. A major application of behaviorism was the behavior modification model in clinical settings. Contemporary behaviorism remains a dominant but diffused force in psychology.