The foundation, accomplishments, and proliferation of behavior therapy have been fueled largely by the movement's grounding in behavioral principles and theories. Ivan P. Pavlov's discovery of conditioning principles was essential to the founding of behavior therapy in the 1950s and continues to be central to modern behavior therapy. Pavlov's major legacy to behavior therapy was his discovery of "experimental neuroses", shown by his students M.N. Eroféeva and N.R. Shenger-Krestovnikova to be produced and eliminated through the principles of conditioning and counterconditioning. In this article, the Pavlovian origins of behavior therapy are assessed, and the relevance of conditioning principles to modern behavior therapy are analyzed. It is shown that Pavlovian conditioning represents far more than a systematic basic learning paradigm. It is also an essential theoretical foundation for the theory and practice of behavior therapy.