Since its introduction in 1938 electroshock therapy (E.C.T.) has become probably the most widely applied form of physical treatment in psychiatry, despite the fact that its method and site of action are still not precisely known. Numerous investigations have been undertaken to delineate more clearly the indications for its use; notwithstanding broad agreement as to its value in depressive states and some schizophrenic syndromes there is still a paucity of evidence on which firm criteria for the prediction of outcome to treatment can be based. Most prognostic studies appeared in the first decade following the introduction of E.C.T. and were based largely on empirical clinical observations during the course of routine psychiatric practice, whereas later investigations have included more intensive and precise researches into possible psychological, physiological, biochemical and pharmacological indices of capacity to respond to electroshock.