Activity of Eosinophilic Esophagitis (EoE) can be measured by patient reported outcomes (symptoms and quality of life) and clinician-reported outcomes (endoscopic, histologic, or biochemical alterations). Over the last years efforts have been underway to develop and validate instruments to assess EoE activity in the different domains. Such instruments are urgently needed to standardize the language of EoE activity assessment and, in so doing, to facilitate communication among various stakeholders. Such standardization will ultimately allow EoE researchers to define meaningful endpoints for use in clinical trials and observational studies, to compare the efficacy of different therapeutic modalities, and to develop algorithms in order to provide patients with the appropriate therapy. This review provides an overview of the current status of instruments that assess EoE activity in the different domains.