Copyright © 2019. All rights reserved. Objective. The OMERACT-Adherence meeting was convened to discuss the conceptual and methodological challenges in developing a core domain set (Adherence-CDS) for trials of interventions for medication adherence in rheumatology. Methods. Forty participants from nine countries participated. Results. Four ideas emerged: for adherence trials, the Adherence-CDS could include adherence and the condition-specific CDS; many factors affect adherence and are intervention targets, contextual factors, or outcome domains; adherence is a critical factor in drug trials; and standardized adherence measures are needed. Conclusion. Despite the challenges, the meeting clarified an approach to developing an Adherence-CDS that complements existing OMERACT work and methodology.