Rheumatoid arthritis (RA) patients and healthcare professionals (HCP) recognize that episodic worsening disease activity, often described as a "flare," is a common feature of RA that can contribute to impaired function and disability. However, there is no standard definition to enable measurement of its intensity and impact. The conceptual framework of the Outcome Measures in Rheumatology Clinical Trials (OMERACT) RA Flare Definition Working Group includes an anchoring statement, developed at OMERACT 9 in 2008: "flare in RA" is defined as worsening of signs and symptoms of sufficient intensity and duration to lead to change in therapy. Subsequently, domains characterizing flare have been identified by comprehensive literature review, patient focus groups, and patient/HCP Delphi exercises. This led to a consensus regarding preliminary domains and a research agenda at OMERACT 10 in May 2010. The conceptual framework of flare takes into account validated approaches to measurement in RA: (1) various disease activity indices (e.g., Disease Activity Score, Clinical Disease Activity Index, Simplified Disease Activity Index); (2) use of patient-reported outcomes (PRO); and (3) characterization of minimally clinically detectable and important differences (MCDD, MCID). The measurement of RA flare is composed of data collection assessing a range of unique domains describing key features of RA worsening at the time of patient self-report of flare, and then periodically for the duration of the flare. The components envisioned are: (1) Patient self-report using a "patient global question"with well characterized and validated anchors; (2) Patient assessment using a flare questionnaire and PRO available at the time of each self-report; (3) Physician/HCP assessment of disease activity status; and (4) Physician's determination whether to change treatment. In randomized controlled trials and observational studies, such a conceptual approach is intended to lead to a valid measure of this outcome/response, thus expanding an understanding of the true impact of a therapy to limit disease activity. Clinically, this approach is intended to enhance patient-HCP communication. This article describes the conceptual framework being used by the OMERACT RA Flare Definition Working Group in developing a standardized method for description and measurement of "flare in RA" to guide individual patient treatment. The Journal of Rheumatology Copyright © 2011. All rights reserved.