INTRODUCTION: Methodological heterogeneity and reporting bias complicate interpreting endodontic outcomes. Supported by the American Association of Endodontists and its Foundation, this study developed the Core Outcome Set in Endodontics (COS-ENDO) to standardize reporting for studies of nonsurgical root canal treatment/retreatment, surgical endodontics, vital pulp therapy, apexification, and regenerative endodontics. This article, the first in a 5-part series, outlines the methods for developing COS-ENDO, with detailed findings for each treatment presented in subsequent parts. METHODS: Outcomes were identified through scoping reviews, a qualitative study with patients, and a web-based survey of American Association of Endodontists practicing members. In a 2-round Delphi survey, clinicians, patients, and researchers rated outcomes on a 9-point scale. Ratings were assessed against consensus criteria, and a virtual meeting finalized COS-ENDO. RESULTS: Of the 97 invited participants to round 1, 77 (79%) registered and 73 (95%) completed the survey, including 24 academicians/researchers, 26 clinicians, and 23 patients (or their caregivers/parents). They rated 64 outcomes, totaling 182 occurrences across treatments: 39 for nonsurgical root canal treatment/retreatment, 46 for surgical endodontics, 46 for vital pulp therapy, and 51 for apexification/regenerative endodontics. The outcomes that achieved consensus for inclusion and those lacking consensus (either for inclusion or exclusion) advanced to round 2 for rerating by 70 participants. During the final consensus meeting, 16 participants discussed the outcomes and finalized the COS-ENDO. CONCLUSION: COS-ENDO is an important step toward standardizing outcome reporting in endodontics. Its implementation will ensure consistent reporting, enhancing research utility and evidence synthesis. Further efforts are needed to establish optimal measurement methods.