This randomized clinical trial will compare the clinical success of two commonly used techniques for performing non-surgical endodontic procedures on teeth with chronic apical periodontitis of pulpal origin. The first method, termed the "GT method" uses a commercially available rotary file system to instrument the root canal systems from the crown to the apical portion of the root. After completion of this technique, the apical portion of the root is prepared to the same standardized ISO size #20 in all roots (i.e., 0.2 mm cross-sectional diameter). The second method, termed the "Lightspeed method" uses a different commercially available rotary file system to shape the root canal systems from the apical to the crown portion of the root. After completion of this technique, the apical preparation is often larger in cross-sectional diameter than that observed after the GT method; however, an equally important distinction is that the Lightspeed method customizes the final cross-sectional diameter for each root canal system, and thus each root has a similar enlargement of apical preparation size due to proportionate removal of infected dentin at the end of the root. Although the Lightspeed method may result in reduced bacteria in the apical portion of root canal systems, there is no evidence to date that this effect improves clinical success rates. This study will test the hypothesis that the larger apical preparation size produced by the Lightspeed instrumentation system will produce a higher percentage of clinically successful treatments of teeth with apical periodontitis (AP) as compared to the GT method