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Eosinophilia, characterized by an absolute eosinophil count ≥ 1.5 x 10^9/L, defines Chronic Eosinophilic Leukemia (CEL), a rare myeloproliferative neoplasm driven by clonal eosinophil precursor proliferation. WHO criteria require persistent eosinophilia, clonality (e.g., ASXL1, TET2 mutations), and exclusion of reactive or neoplastic causes. Eosinophil infiltration into skin (most common), lungs (44%), gastrointestinal tract (38%), and heart leads to severe complications like endomyocardial fibrosis, contributing to a poor prognosis with ~22-month median survival. Molecular and cytogenetic testing is essential to distinguish CEL from idiopathic hypereosinophilic syndrome and other malignancies, enabling targeted management to reduce organ damage.