Current, relevant knowledge on the outcome of endodontic therapy is key to clinical decision making, particularly when endodontic treatment is weighed against tooth extraction and replacement. Inherent to reviewing the outcome is a definition of "success" in relation to the goals of therapy. As the specific goal set out by the individual patient may either be healing/prevention of disease (apical periodontitis) or just functional retention of the tooth, the potential for both healing and functionality is reviewed. Based on selected follow-up studies that offer the best evidence, the chance of teeth without apical periodontitis to remain free of disease after initial treatment or orthograde retreatment is 92 percent to 98 percent. The chance of teeth with apical periodontitis to completely heal after initial treatment or retreatment is 74 percent to 86 percent, and their chance to be functional over time is 91 percent to 97 percent. Thus there does not appear to be a systematic difference in outcome between initial treatment and orthograde retreatment. The outcome of apical surgery is less consistent than that of the nonsurgical treatment. The chance of teeth with apical periodontitis to completely heal after apical surgery is 37 percent to 85 percent, with a weighted average of approximately 70 percent. However, even with the lower chance of complete healing, the chance for the teeth to be functional over time is 86 percent to 92 percent. Considering the favorable outcome, conservative endodontic therapy, both nonsurgical and surgical, is definitely justified and should be attempted when a good restorative and periodontal prognosis is projected, unless the patient is not motivated to retain the tooth.