Hypereosinophilic syndrome (HES) is characterized by chronic unexplained eosinophilia with organ involvement. The concept of HES as a single disease entity is being challenged by the recent identification of multiple underlying molecular mechanisms. HES can directly affect the eosinophil lineage (often linked to a fusion gene FIP1L1-PDGFRA, and corresponding in this case to chronic eosinophilic leukemia), or the lymphoid lineage, where eosinophilia is secondary to expansion of a T cell subset overproducing interleukin-5, a cytokine involved in eosinophilopoiesis. These recent discoveries have legitimized the use of tyrosine kinase inhibitors such as imatinib, which, by inhibiting PDGFRA, have transformed the prognosis of chronic eosinophilic leukemia, and also the use of monoclonal anti-IL-5 antibodies, which are promising treatment for steroid-dependent HES.