The mast cell is a virtual pharmacopoeia of biological substances. It used to be believed that mast cell activation was all-or-nothing, with IgE cross-linking inducing symptoms of allergy and anaphylaxis. However, the activity of mast cells in health and disease is clearly much more complicated than this. The discovery that human mast cells secrete many pleiotropic cytokines suggested there may be many novel mast cell functions, and many of these are now being realised. The ubiquitous distribution of mast cells throughout connective tissues, along epithelial surfaces, and in close proximity to blood vessels, makes their products available to a large variety of cell types including fibroblasts, glandular epithelial cells, nerves, vascular endothelial cells, smooth muscle cells, and other cells of the immune system. This tissue distribution, and the vast array of lipid mediators, proteases, proteoglycans and cytokines identified as potential products of human mast cells, explains how this interesting cell has the potential to contribute to so many diverse biological events.