The receptor tyrosine kinase c-Kit and its ligand Stem Cell Factor (SCF) are essential for haemopoiesis, melanogenesis and fertility. SCF acts at multiple levels of the haemopoietic heirarchy to promote cell survival, proliferation, differentiation, adhesion and functional activation. It is of particular importance in the mast cell and erythroid lineages, but also acts on multipotential stem and progenitor cells, megakaryocytes, and a subset of lymphoid progenitors. SCF exists in soluble or transmembrane forms which appear to differ in function. Multiple isoforms of c-Kit also exist as a result of alternate mRNA splicing, proteolytic cleavage and the use of cryptic internal promoters in certain cell types. This review focuses on what is known about the regulation of c-Kit expression, the functions of SCF and c-Kit isoforms, and the nature of the biological responses elicited by this receptor-ligand pair with emphasis on the haemopoietic system.