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Hypereosinophilic syndrome consists of a group of disorders characterized by abnormal accumulation of eosinophils in the blood or peripheral tissues, independent of known secondary causes of eosinophilia such as parasitic infection. Clinical manifestations of the condition are highly variable, ranging from asymptomatic eosinophilia to severe tissue damage and end-organ failure. This entity has been recognized for decades, with early studies identifying distinct groups of patients with differing symptoms, exam findings, laboratory abnormalities, and prognosis. In the past, these patients were treated with non-targeted immunosuppressive agents, often with limited efficacy. More recently, advances in the knowledge of eosinophil biology and molecular diagnostics have allowed for more specific delineation of the many disease subgroups that characterize hypereosinophilic syndrome. Identification of these groups has led to a personalized management approach to the condition, with improved diagnostic techniques as well as stratification of patients into more effective treatment groups. This review will discuss the evolution of the definition of hypereosinophilic syndrome, outline current disease classifications, provide a guide for evaluation and monitoring, and discuss current and future therapeutic modalities.