This lecture covers entities within the group of MDS/MPN overlap, including general principles of these diseases and their distinction from the 'pure' myelodysplastic syndromes and myeloproliferative neoplasms. The lecture also covers myeloid/lymphoid neoplasms with eosinophilias and gene rearrangement and their differential diagnosis with reactive eosinophilias, chronic eosinophilic leukemia, and idiopathic hypereosinophilic syndrome. The MDS/MPN and eosinophilic neoplasms are some of the most challenging diagnoses in hematopathology. The lecture is appropriate for hematopathology fellows, residents who have already had exposure to diagnostic hematopathology, and practicing pathologists.