Hypereosinophilic syndrome (HES) includes heterogeneous hematological disorders that are characterized by distinctive blood and tissue eosinophilia. In addition to classical HES criteria, the World Health Organization proposed a set of criteria that distinguish chronic eosinophilic leukemia (CEL) from HES. As such, the fusion gene FIP1L1/PDGFRA was found as a cause of CEL in a significant proportion of patients initially diagnosed as having HES. Several investigations have tried to dissect the mechanism of leukemogenesis; eosinophilia and signaling induced by FIP1L1/PDGFRα in cell lines, bone marrow mast cells, primary human eosinophils and in murine myeloproliferative disorder models. In this review, we introduce the current knowledge on the relationship between FIP1L1/PDGFRα and cell signaling, eosinophil proliferation, survival and activation and mastocytosis specially focusing on the evidence learned from murine models.