Mast cells have gained increased recognition as immunomodulators playing a role in a variety of physiological and pathological processes. They were first described in 1879, but their origin remained controversial for almost a century. Today, it is known that mast cells are present in the bone marrow as committed mast cell precursors. They leave the bone marrow as progenitors and complete their maturation at peripheral sites. Investigations on the maturation of bone marrow derived mast cells focused on bone marrow cultured in the presence of interleukin-3 (IL-3) and stem cell factor (SCF). SCF is essential for mast cell survival and mice that lack either SCF or the receptor for SCF are mast cell deficient. It is the microenvironment surrounding the mast cell that determines its mature phenotype. SCF, IL-3 and IL-9 have been identified among the most important cytokines for regulation of mast cell growth and differentiation. Several factors have been identified as chemoattractants for mast cells, but their exact mechanism of action remains unclear. Mast cell recruitment is most likely a combination of the direct effect of mast cell mediators on the mast cell progenitor as well as the indirect effect of these mediators on other cell types.