The need for an agreement of what constitutes success or failure in endodontic therapy in humans is emphasized. Once such agreement has been reached, major prospective studies can be undertaken in which clinical variables will be related to success or failure. Valid retrospective studies relating technique or material variables to success or failure may also be possible, if access were to be gained to a sufficient data base. Such studies would be of worth in themselves, but perhaps more importantly would then allow for correlation between in vitro laboratory studies of the same variables and clinical success or failure. Until such clinical studies are performed, the great majority of laboratory investigations into endodontic materials will be of doubtful value.