The tissue hormone stem cell factor (SCF) is a major and unique agonist for human mast cells. This stromal cell derived cytokine regulates the differentiation, growth, survival, location, cytokine production, mediator secretion, and chemotaxis of human mast cells. The effects of SCF are mediated through the c-kit proto-oncogene product, a tyrosine kinase receptor expressed on mast cells and mast cell progenitor cells. The present article gives an overview on the role of c-kit and c-kit ligand in the biology of human mast cells, and briefly outlines some arising pathophysiologic concepts.