Subjects admitted on this protocol will have elevated eosinophil counts in the peripheral blood or tissues or will be relatives of subjects with eosinophilia. Eosinophilic subjects will undergo an extensive clinical evaluation focused on the identification of the cause of eosinophilia and the presence of end organ manifestations. In addition, they will be characterized in detail immunologically, and their blood cells and/or serum will be collected to provide reagents (eg. specific antibodies, T-cell clones, etc.) that will be used in the laboratory to address broader questions relating to the etiology of eosinophilia, its immunoregulation, the degree and source of eosinophil activation, and/or the functional role of eosinophils in the afferent arm of those immune response where they are prominent. While the protocol is not primarily designed to study treatment of patients with blood and tissue eosinophilia, the clinical and immunological responses to various medically indicated therapies will be carefully monitored. The subjects themselves will be followed over time to determine the kinetics and nature of the factors affecting their degree of eosinophilia and the level of activation or degranulation of the eosinophils at various time points and in response to various stimuli. It is anticipated that the subjects will receive a degree of clinical evaluation, care and monitoring more extensive than that generally available and that the specimens collected from them will prove to be valuable reagents for laboratory studies related to eosinophilia, eosinophil activation and function. Recognized causes of subjects hypereosinophilia will be treated appropriately either by our own clinical service or by the referring physicians. Standard of care therapy may be provided when indicated for HES variants. This protocol will also allow clinical and laboratory evaluation of family members of subjects with eosinophilia in order to help identify genetic causes of eosinophilia and to provide controls for immunologic studies. Interviews will be conducted on a subset of HES patients to provide information about patient signs and symptoms for use in the development of a patient-related outcomes questionnaire for use in future treatment studies. Finally, adult subjects with gastrointestinal involvement may be asked to provide stool samples for microbiome analysis and assessment of stool levels of eosinophil granule proteins to explore the role of the microbiome in eosinophilic gastrointestinal disease. Stool samples may also be obtained from adult subjects undergoing endoscopy and/or colonoscopy as part of their initial evaluation and/or assessment of response to treatment. Potential confounding variables, including body mass index, diet, stool transit time and vitamin D levels will be assessed at the time of stool collection.