Increasingly, endodontists are reaching  a  consensus on the criteria for acceptable and unacceptable endo­ dontic treatment (Table I). The reasons for endodontic successes and failures have been reported in the litera­ ture (Figs. 1 and 2).-1 20 The evaluation of previous root canal therapy is discussed in this article and guidelines proposed for the retreatment of endodontic failures.