Mast cells are widely distributed throughout the body in both connective tissue and at mucosal surfaces, and form a heterogeneous population of cells with differences apparent in their mediator content, ultrastructure and functional behaviour. This applies both within the same tissue and between different tissues (reviewed in [1]). They arise from bone marrow-derived progenitor cells that circulate as undifferentiated CD34+ mononuclear cells in the peripheral circulation (reviewed in [2]). These progenitors subsequently migrate into tissue and mature under the influence of locally derived growth factors and cytokines, particularly stem cell factor. Mast cells are therefore found in significant numbers in normal airways, located predominantly in the subepithelial compartment, particularly adjacent to blood vessels.