The model genetic organism Drosophila melanogaster has a rudimentary hematopoietic system with two embryonic blood cell types, crystal cells, and plasmatocytes. These distinct lineages provide the animal with an innate immune response and a means to remove apoptotic cells. Genetic analyses of Drosophila hematopoiesis have identified specific genes that function in blood cell formation. Complimentary deoxyribonucleic acid and antibody probes for these hematopoietic factors serve as important reagents to follow hemocyte lineage commitment and differentiation during embryogenesis. This is possible through methods described in this chapter on messenger ribonucleic acid localization by in situ hybridization and protein localization by immunohistochemical staining within hemocyte precursors and mature blood cells.