The Journal of Rheumatology Copyright © 2019. All rights reserved. Objective. The current Juvenile Idiopathic Arthritis (JIA) Core Set used in randomized controlled trials (RCT) and longitudinal observational studies (LOS) was developed without the input of patients/parents. At the Outcome Measures in Rheumatology (OMERACT) 2016, a special interest group voted to reconsider the core set, incorporating broader input. We describe subsequent work culminating in an OMERACT 2018 plenary and consensus voting. Methods. Candidate domains were identified through literature review, qualitative surveys, and online discussion boards (ODB) held with patients with JIA and parents in Australia, Italy, and the United States. A Delphi process with parents, patients, healthcare providers, researchers, and regulators served to edit the domain list and prioritize candidate domains. After the presentation of results, OMERACT workshop participants voted, with consensus set at > 70%. Results. Participants in ODB were 53 patients with JIA (ages 15–24 yrs) and 55 parents. Three rounds of Delphi considering 27 domains were completed by 190 (response rate 85%), 201 (84%), and 182 (77%) people, respectively, from 50 countries. There was discordance noted between domains prioritized by patients/parents compared to others. OMERACT conference voting approved domains for JIA RCT and LOS with 83% endorsement. Mandatory domains are pain, joint inflammatory signs, activity limitation/physical function, patient’s perception of disease activity (overall well-being), and adverse events. Mandatory in specific circumstances: inflammation/other features relevant to specific JIA categories. Conclusion. Following the OMERACT methodology, we developed an updated JIA Core Domain Set. Next steps are to identify and systematically evaluate best outcome measures for these domains.