Eosinophil differentiation is a complex series of events regulated by cytokines at multiple levels, including proliferation, survival, and maturation. The development of an ex vivo eosinophil differentiation model, using the current knowledge on factors involved in this process, has facilitated efforts to understand the molecular mechanisms underlying human eosinophil development. Differentiation of human hematopoietic progenitor cells, isolated by density centrifugation and immunomagnetic cell separation, towards mature eosinophils, involves a 17-day culture period in the presence of a mixture of cytokines. At early stages of differentiation, these cells can be retrovirally transduced resulting in modulation of the expression of genes of interest to examine their role in eosinophil development. Eosinophil maturation can be analyzed by combining three different methods: histochemical analysis, flow cytometric analysis, and Luxol Fast Blue staining. In addition to this ex vivo differentiation model, human hematopoietic progenitors can be transplanted into immune-deficient mice resulting in the development of all human hematopoietic lineages in the mouse bone marrow, including eosinophils. Although the ex vivo differentiation model can be used separately, combining it with the transplantation model will give insight into not only regulation of human eosinophil development but also hematopoiesis in general.