The proliferation and differentiation of macrophages is controlled by CSF1R and its two ligands, CSF1 and IL34. In adults, CSF1R is expressed exclusively in the macrophage lineage. CSF1R-dependent macrophages have numerous trophic roles in development and homeostasis. CSF1R-deficient mice are severely compromised, and few survive to weaning. Anti-CSF1R antibody administration to adult mice depleted the large majority of tissue macrophages, and impacts indirectly on intestinal differentiation. We have produced a long-acting form of CSF1, CSF1-Fc, that greatly expands tissue macrophage populations. The effect of CSF1-Fc administration revealed the essential function of Kupffer cells in homeostatic regulation of the size of the liver. The molecular basis for transcriptional regulation of Csf1r in macrophages has been studied in detail. We have used conserved elements within the gene to produce reporter genes in mice, rats, chickens and sheep. One intronic element, called FIRE, is conserved from reptiles to humans. We have deleted this element in the mouse germ line. Surprisingly, the FIRE-/- mice lack selected macrophage populations, including microglia, Langerhans cells and peritoneal macrophages, but are healthy and fertile. Other conserved elements appear to be required for specific macrophage populations in mice. We have also knocked out the Csf1r gene in rats. By contrast to mice, the Csf1r-/- rats survive to adulthood, revealing additional pleiotropic roles of CSF1R-dependent macrophages. The species-specific differences in CSF1R biology may arise from differential regulation of the ligand, CSF1, in macrophages.