Eosinophilic Esophagitis (EoE) is a chronic inflammatory disease of the esophagus affecting children and adults. The most frequent symptoms are swallowing difficulties and thoracic pain. The disease has first been described in the 1980s and is found with rapidly increasing frequency mainly in industrialized countries. The factors that lead to EoE are until now incompletely understood, of importance, the disease is found more frequently in men and patients suffering from allergic diseases (e.g., Asthma). So far there exists no activity index to define the severity of EoE; such an index is urgently needed for future clinical trials to determine the efficacy of current and future therapies. The investigators' 3-year project, carried out in cooperation with international EoE experts, aims at the development of an activity index for adult and pediatric EoE patients that will be used in future clinical trials as well as observational studies.