Joseph Wolpe brought about a revolution in psychotherapy. He provided the first clear alternative to therapy as an esoteric exploration of mental forces and presented, instead, clearly specified procedures and documented outcomes - a practical technology based on a fundamental science of learning. Roger Poppen, who witnessed some of this revolution first hand as a doctoral student, describes the major impact Wolpe's theories had on psychotherapy, compelling it to address issues of effectiveness and accountability. He assesses the criticisms that Wolpe's work has attracted both from outside and within the behavioural school, and describes the development of Wolpe's ideas and his continuing role in the theory, practice and evaluation of psychotherapy.