The lack of core outcome sets for atopic eczema (AE) is a major obstacle for advancing evidence-based treatment. The global Harmonising Outcome Measures for Eczema (HOME) initiative has already defined clinical signs, symptoms, quality of life, and long-term control of flares as core outcome domains for AE trials. This article deals with the standardization of measurement instruments to assess clinical signs of AE. To resolve the current lack of standardization of the assessment of clinical signs of AE, we followed a structured process of systematic reviews and international consensus sessions to identify 1 core outcome measurement instrument for assessment of clinical signs in all future AE trials. Systematic reviews indicated that from 16 different instruments identified to assess clinical signs of AE, only the Eczema Area and Severity Index (EASI) and the objective Scoring Atopic Dermatitis (SCORAD) index were identified as extensively validated. The EASI has adequate validity, responsiveness, internal consistency, and intraobserver reliability. The objective SCORAD index has adequate validity, responsiveness, and interobserver reliability but unclear intraobserver reliability to measure clinical signs of AE. In an international consensus study, patients, physicians, nurses, methodologists, and pharmaceutical industry representatives agreed that the EASI is the preferred core instrument to measure clinical signs in all future AE trials. All stakeholders involved in designing, reporting, and using clinical trials on AE are asked to comply with this consensus to enable better evidence-based decision making, clearer scientific communication, and improved patient care.