Ivan P. Pavlov was awarded the Nobel Prize for Physiology or Medicine in December, 1904 “in recognition of his work on the physiology of digestion”. By this time Pavlov had started to study “conditioned reflexes” and described how he came to study “psychical reflexes” in his “Madrid speech” (April 1903). In 1906, Pavlov [spelled Pawlow] gave the Huxley Lecture, entitled “The scientific investigation of the psychical faculties or processes in the higher animals” which was published in The Lancet (6 Oct. 1906). One of the earliest American proponents of classical conditioning was John B. Watson, the founder of “Behaviorism”. Watson conducted experiments on conditioned motor reflexes using the method of Bechterev and conducted his most infamous experiment on “Conditioned emotional reactions” on “little Albert” in 1920. Karl S. Lashley did early work on conditioned salivary reflexes in humans. Pavlov visited America in 1923 and 1929 and his Lectures on conditioned reflexes were published in English in 1927 and 1928. Pavlov provided American psychologists with an objective philosophical approach as opposed to subjective methods such as introspection; and a methodological approach through the use of conditioned reflexes. While these were embraced by Americans, the theoretical approach to the study of higher nervous activity through conditioned reflexes, was not so readily embraced. Although Pavlov’s methods of conditioned reflexes influenced the theories of Edwin Guthrie, B.F. Skinner, Clark Hull and Kenneth Spence, Guthrie and Lashley (1930) were critical of Pavlov’s theory of neural function. Pavlov wrote a scathing reply to these critiques (1932). Pavlov’s student, Boris Babkin influenced Donald Hebb to study conditioned reflexes and Hebb went on to develop a neurophysiological theory that integrated the ideas of Pavlov and Sherrington on reflexes with the theories of Lashley, Kohler and Tolman into a coherent theory that provided a neural mechanism that explained perception, attention, learning and memory. Hebb developed three ideas to provide a “modern” view of the neural changes underlying cognitive function: (1) the “Hebb synapse” or “Hebb learning rule”; (2) the cell assembly and (3) the phase sequence. Today Pavlovian conditioning is used to study the neurobiology of learning and memory at four levels: (1) behavioural; (2) neural systems and circuits; (3) cellular mechanisms of synaptic plasticity; and (4) molecular mechanisms of neural plasticity underlying learning and memory. Of note are the work of LeDoux, Faneslow and others on the neural circuits of conditioned fear and the work of Thompson on the role of the cerebellum in conditioned motor responses. Pavlovian conditioning is also used to understand abnormal behaviour and to develop clinical methods of behaviour therapy. Thus, much of the history of Psychology and Neuroscience in America has been influenced by the concepts of classical conditioning as developed by Pavlov, criticized by Lashley and incorporated into Hebb’s neurophysiological theory.