Over the thirty years since Strindberg's comprehensive study, much effort has been invested in an attempt to formulate a clear-cut answer to the basic question: What is the success rate in endodontics? The discrepancy and disagreement as reported in the endodontic literature with regard to success rate and the influence of the various factors on treatment results, prompted us to engage in a thorough analysis of the major causes for the variability of the reported data. These causes are related to the design of the studies, the endodontic techniques employed in them, the qualifications of the operators performing the treatment and the complexity of the included cases. They are also related to the observation period in each study, as well as to the criteria that were used for evaluating the treatment results.