Core outcome sets are critically important outcomes that should be measured in clinical trials. Their absence in atopic dermatitis is a form of research waste and impedes combining evidence to inform patient care. Here, we articulate the rationale for core outcome sets in atopic dermatitis and review the work of the international Harmonising Outcome Measures for Eczema group from its inception in Munich, 2010. We describe core domain determination (what should be measured), to instrument selection (how domains should be measured), culminating in the complete core outcome measurement set in Tokyo, 2019. Using a "road map," Harmonising Outcome Measures for Eczema includes diverse research methods including Delphi and nominal group techniques informed by systematic reviews of properties of candidate instruments. The 4 domains and recommended instruments for including in all clinical trials of atopic dermatitis are patient symptoms, measured by Patient-Oriented Eczema Measure and peak Numerical Rating Scale 11 for itch intensity over 24 hours, clinical signs measured using the Eczema Area and Severity Index, quality of life measured by the Dermatology Life Quality Index series for adults, children, and infants, and long-term control measured by either Recap of atopic eczema or Atopic Dermatitis Control Tool.