Background. While disease flares in rheumatoid arthritis (RA) are a recognized aspect of the disease process, there is limited formative research to describe them. METHODS: The Outcome Measures in Rheumatology Clinical Trials (OMERACT) RA Flare Definition Working Group is conducting an international research project to understand the specific characteristics and impact of episodic disease worsening, or "flare," so that outcome measures can be developed or modified to reflect this uncommonly measured, but very real and sometimes disabling RA disease feature. Patient research partners provided critical insights into the multidimensional nature of flare. The perspectives of patients and healthcare and research professionals are being integrated to ensure that any outcome measurement to detect flares fulfills the first OMERACT criteria of Truth. Through an iterative data-driven Delphi process, a preliminary list of key domains has been identified to evaluate flare. RESULTS: At OMERACT 10, consensus was achieved identifying features of flare in addition to the existing core set for RA, including fatigue, stiffness, symptom persistence, systemic features, and participation. Patient self-report of flare was identified as a component of the research agenda needed to establish criterion validity for a flare definition; this can be used in prospective studies to further evaluate the Discrimination and Feasibility components of the OMERACT filter for a flare outcome measure. CONCLUSION: Our work to date has provided better understanding of key aspects of the RA disease process as episodic, potentially disabling disease worsening even when a patient is in low disease activity. It also highlights the importance of developing ways to enhance communication between patients and clinicians and improve the ability to achieve "tight control" of disease.