This presentation will address an approach to the diagnosis of the myeloid neoplasms in which eosinophilia is a significant component. In these disorders, the eosinophils belong to the malignant clone, although this is difficult to confirm with routine tests. There is no specific immunophenotypic marker to identify these clonal eosinophils, and the classification is based on the recognition of the underlying genetic abnormality. The best described fusion genes involve PDGFRA (platelet-derived growth factor receptor, alpha polypeptide), PDGFRB (PDGFR, beta polypeptide) and FGFR1 (fibroblast growth factor receptor 1), as described in the WHO 2008 classification. The FIP1L1-PDGFRA fusion gene is the most frequent recurrent aberration, and this entity is important to recognise as it shows exquisite sensitivity to tyrosine kinase inhibitors. In cases with rearrangements of PDGFRB the haematological features are most commonly those of CMML, and although eosinophilia is common it is not invariable. Haematologic neoplasms with FGFR1 rearrangements are heterogeneous and are associated with a poor prognosis; the presentation may be as a myeloproliferative neoplasm or acute leukaemia. Other disorders associated with eosinophilia including acute myeloid leukaemia, chronic eosinophilic leukaemia and systemic mastocytosis will be briefly discussed.