Macrophages, which are heterogeneous populations existing in various tissues and organs, are responsible for numerous metabolic, immunological, and inflammatory processes in physiological and pathological conditions. Macrophage heterogeneity is observed from early ontogeny. Primitive macrophages first develop in yolk sac hematopoiesis, by-passing the differentiation pathway of the monocytic series to differentiate into fetal macrophages in various tissues. Monocytic cells are a minor cell population in the early fetal period, and increase in the late stage. Primitive/fetal macrophages proliferate and survive in loco in the fetal period and reside as resident macrophages after birth. In adult animals, monocytes are differentiated from promonocytes derived from pluripotent stem cells in bone marrow. Monocytes exude in inflammatory foci and differentiate into exudate macrophages, exudateresident macrophages and peroxidase-negative macrophages, but not resident macrophages. Monocyte-derived macrophages are a short-lived and non-proliferating cell population. Tissue (resident) macrophages proliferate and maintain their population by self renewal. In mice rendered monocytopenic by administration of a bone-seeking isotope, strontium-89, tissue resident macrophages maintained their population for 6 weeks. Resident macrophages in the liver formed glucan-induced granulomas in this monocytopenic model. In macrophage colony stimulating factor (M-CSF)-deficient mice (op/op), monocytes as well as tissue macrophages are deficient. However, M-CSF-independent tissue macrophages and Langerhans/dendritic cells are present in the defective condition of monocyte differentiation into macrophages, indicating that differentiation pathways of tissue macrophages and nonlymphoid dendritic cells are different from those of monocytes. In cultures supplemented with various colony stimulating factors (CSFs), heterogenous macrophage populations were generated. These in vivo and in vitro findings suggest that the phenotypic and functional heterogeneity of macrophages reflects complex macrophage differentiation mechanisms and that CSFs are important factors in the formation of a microenvironment for macrophage differentiation.